Method of biotreatment for solid materials in a nonstirred surface bioreactor

ABSTRACT

A method of biotreating and recovering metal values from metal-bearing refractrory sulfide ore using a nonstirred surface bioreactor is provided. According to the method, the surface of a plurality of coarse substrates is coated with a solid material to be biotreated to form a plurality of coated coarse substrates. The coarse substrates have a particle size greater than about 0.3 cm and the solid material to be biotreated has a particle size less than about 250 μm. A nonstirred surface reactor is then formed by stacking the plurality of coated coarse substrates into a heap or placing the plurality of coated coarse substrates into a tank so that the void volume of the reactor is greater than or equal to about 25%. The solid material is biotreated in the surface bioreactor until the undesired compound in the solid material is degraded to a desired concentration.

PRIORITY INFORMATION

This application is a divisional of U.S. patent application Ser. No.10/971,331, filed Oct. 21, 2004, now U.S. Pat. No. 7,416,882, which is acontinuation of U.S. patent application Ser. No. 10/176,854, filed Jun.20, 2002, now U.S. Pat. No. 6,855,527, which is a continuation of U.S.patent application Ser. No. 09/735,156, filed Dec. 12, 2000, now U.S.Pat. No. 6,410,304, which is a continuation of Ser. No. 09/097,316,filed Jun. 12, 1998, now U.S. Pat. No. 6,159,726, which is acontinuation of U.S. patent application Ser. No. 08/636,117, filed Apr.22, 1996, now U.S. Pat. No. 5,766,930, which is a continuation-in-partof U.S. patent application Ser. No. 08/588,589, filed Jan. 18, 1996, nowU.S. Pat. No. 6,083,730, which is a continuation-in-part of U.S. patentapplication Ser. No. 08/459,621, filed Jun. 2, 1995, now abandoned. Eachof the foregoing applications is incorporated herein by reference as iffully set forth.

TECHNICAL FIELD

The present invention relates to the biotreatment of solid materials. Inparticular, the present invention relates to the ex situ biotreatment ofsolid materials in an aerobic process to degrade an undesired compoundpresent in the solid material.

BACKGROUND ART

Biological treatment processes are finding application throughoutindustry. Such processes have been used in waste water treatment,hazardous waste remediation, desulfurization of coal, and biooxidationof refractory sulfide ores.

A variety of methods can be employed in the biological treatment ofsolid materials, including in situ treatment, landfarming, composting,heap treatment, and stirred or agitated tanks. In the ex situ biologicaltreatment of solid materials, some sort of bioreactor is used to carryout the biotreatment. A bioreactor can be defined as a vessel or body inwhich biological reactions are carried out by microorganisms, or enzymesthey produce, contained within the reactor itself. The main objective inthe design of a bioreactor is to generate an optimal environment for thedesired biological process to take place on a large and economic scale.

When a solid material is being biotreated, the desired biologicalreactions typically involve the degradation, either directly orindirectly, of some undesired compound present in the solid material. Toaccomplish this economically, the bioreactor needs to reduce theconcentration of the undesired compound to an acceptable level in anacceptable quantity (in terms of flow rate) of solid material to betreated.

In general biotreatment processes are slow, and if they are aerobic,they require large amounts of oxygen for the aerobic microorganism(s) tometabolize, either directly or indirectly, the undesired compound.Oxygen transfer, therefore, is typically a major problem for the largeclass of aerobic biological treatment processes available. Currentaerobic bioreactor designs attempt to ensure not only that themicroorganisms being used have access to the material to be biooxidizedor metabolized, but also that all areas of the bioreactor have anadequate oxygen and nutrient supply, as well as maintain the correct pHand temperature, for the biological process to proceed.

Stirred tank bioreactors are used in many types of aerobic biologicalprocesses, including biooxidation of refractory sulfide gold ores andbioremediation of contaminated soils. Stirred tank bioreactors providevery good contact between the bioleachant and the solid material to betreated. In addition, stirred tank processes typically have favorableoxygen conditions because the tank is sparged with air or oxygen.However, even in stirred tank bioreactors where oxygen is provided byair or oxygen sparging, the low solubility of oxygen in water (10 ppm)requires a large gas-water interface. This is generally achieved withimpellers and significant expenditures of energy. The high energy costsassociated with stirring and aerating the reactor make this type ofbioreactor primarily applicable to bioprocesses that come to a desiredend point relatively quickly, typically less than a week. For slowerbiological processes, a low energy cost, large scale, generally staticbatch process, is the best solution. However, the goal of providing thebacteria, or other microorganism, with an optimal environment is stillof primary importance.

There are three primary types of static batch bioreactors used tobiotreat soils contaminated with toxic organic compounds. One of thesemethods is landfarming. This is an above grade treatment of contaminatedsoil in a large open space. The soil is spread over a high-densitypolyurethane lined area generally covered with sand to allow fordrainage. Air can be introduced by perforated pipes and by tilling thesoil once or twice a week. This method has been widely implemented atsites contaminated with polynuclear aromatic (PNA's) andpentachlorophenol (PCP). One limitation of this process is that a largearea is needed because the soil is spread relatively thinly to ensureadequate air flow. This method also requires tilling and may be limitingin air if the layer of soil is too thick or does not mix well.

Another technology used in the bioremediation of contaminated soil iscomposting. The compost is made up of contaminated soil and variousamendments necessary for composting to be sustained such as wood chips,straw, or manure. These amendments increase the amount of biodegradableorganics, structurally improve the compost matrix by reducing bulkweight and increasing air voids, and increase the amount of inorganicnutrients in the mixture. The composting can be carried out in a vesselwith forced air flow or in open piles that are aerated by air pipes orby tilling. One disadvantage to the addition of organic amendments isthat their biodegradation generates heat and requires oxygen. Compostingis usually run in batch mode and a portion of the compost is used toinoculate the next compost. This process has been used effectively onmany types of organic contaminates including diesel fuel, 2,4,6trinitrotoluene (TNT), polyaromatic hydrocarbons (PAH), benzene, andxylene.

Heap bioremediation is another static bioprocess used in thebioremediation of excavated contaminated soil. In this process the soilis placed in piles 8 to 12 feet high over a lined area. To improve airflow, air can be introduced by perforated pipes. In such circumstances,the pipes are placed on approximately a 12 inch bed of the contaminatedsoil in regular intervals. The pipes are then typically covered with alayer of gravel to protect them from the heavy equipment. The excavatedsoil is then dumped in an 8 to 12 foot high pile on top of the gravel.Moisture is maintained with an irrigation system. The soil may needfertilizer or lime to adjust pH and may need sand to increase porosity.This process is low cost and thus is applicable to slow biologicalprocesses. However, this process may be too slow if the heap becomes airlimited due to compaction of the soil during or after pile construction.

Therefore, air and liquid access remain important rate limitingconsiderations in existing static batch bioprocesses used for soilremediation, such as heap pile bioremediation, composting andlandfarming. Air flow is improved in existing processes to the extentpossible by introducing air through perforated air pipes or by tillingthe soil. However, any flow constriction within the bioreactor willinterfere with the efficiency of the process. Also, if parts of thecontaminated soil are not exposed to bacteria or other nutrients as wellas oxygen, the overall bioprocess will be slowed or not proceed tocompletion. Similarly, in the case of heap biooxidation of coal andrefractory sulfide gold ore, biooxidation of the sulfides is efficientlycarried out by the bacteria only when the metal sulfides are exposed tobacteria, water, nutrients, and air. If the sulfides are buried in theore or in the solid pieces of coal, the biooxidation will not proceed.In addition, if air or liquid flow in the heap becomes limited, thebiooxidation will also become limited. Consequently, a need exists foran improved bioreactor design that will permit the biotreatment of solidmaterials with improved air and fluid flow throughout the bioreactor andthe solid material to be treated.

The use of acidophilic, autotrophic bacteria to biooxidize sulfideminerals in refractory sulfide ores is one biotreatment that has gainedparticular vigor in the last ten to twenty years.

Gold is one of the rarest metals on earth. Gold ores can be categorizedinto two types: free milling and refractory. Free milling ores are thosethat can be processed by simple gravity techniques or directcyanidation. Refractory ores, on the other hand, are not amenable toconventional cyanidation treatment. Gold bearing deposits are deemedrefractory if they cannot be economically processed using conventionalcyanide leaching techniques because insufficient gold is solubilized.Such ores are often refractory because of their excessive content ofmetallic sulfides (e.g., pyrite and arsenopyrite) and/or organiccarbonaceous matter.

A large number of refractory ores consist of ores with a precious metalsuch as gold occluded in iron sulfide particles or other metal sulfideparticles. The iron sulfide particles consist principally of pyrite andarsenopyrite. Precious metal values are frequently occluded within thesulfide mineral. For example, gold often occurs as finely disseminatedsub-microscopic particles within a refractory sulfide host of pyrite orarsenopyrite. If the gold, or other precious metal, remains occludedwithin the sulfide host, even after grinding, then the sulfides must beoxidized to liberate the encapsulated precious metal values and makethem amenable to a leaching agent (or lixiviant); thus, the sulfideoxidation process reduces the refractory nature of the ore.

A number of processes for oxidizing the sulfide minerals to liberate theprecious metal values are well known in the art. These methods cangenerally be broken down into two types: mill operations and heapoperations. Mill operations are typically expensive processes havinghigh operating and capital costs. As a result, even though the overallrecovery rate is typically higher for mill type processes, milloperations are typically not applicable to low grade ores, that is oreshaving a gold concentration less than approximately 0.07 oz/ton. Milloperations are even less applicable to ores having a gold concentrationas low as 0.02 oz/ton.

Two well known methods of oxidizing sulfides in mill type operations arepressure oxidation in an autoclave and roasting.

Oxidation of sulfides in refractory sulfide ores can also beaccomplished using acidophilic, autotrophic microorganisms, such asThiobacillus ferrooxidans, Sulfolobus, Acidianus species andfacultative-thermophilic bacteria in a microbial pretreatment. Thesemicroorganisms utilize the oxidation of sulfide minerals as an energysource during metabolism. During the oxidation process, the foregoingmicroorganisms oxidize the iron sulfide particles to cause thesolubilization of iron as ferric iron, and sulfide, as sulfate ion.

If the refractory ore being processed is a carbonaceous sulfide ore,then additional process steps may be required following microbialpretreatment to prevent preg-robbing of the aurocyanide complex or otherprecious metal-lixiviant complexes by the native carbonaceous matterupon treatment with a lixiviant.

As used herein, sulfide ore or refractory sulfide ore will be understoodto also encompass refractory carbonaceous sulfide ores.

A known method of bioleaching carbonaceous sulfide ores is disclosed inU.S. Pat. No. 4,729,788, issued Mar. 8, 1988, which is herebyincorporated by reference. According to the disclosed process,thermophilic bacteria, such as Sulfolobus and facultative-thermophilicbacteria, are used to oxidize the sulfide constituents of the ore. Thebioleached ore is then treated with a blanking agent to inhibit thepreg-robbing propensity of the carbonaceous component of the ore. Theprecious metals are then extracted from the ore using a conventionallixiviant of cyanide or thiourea.

Another known method of bioleaching carbonaceous sulfide ores isdisclosed in U.S. Pat. No. 5,127,942, issued Jul. 7, 1992, which ishereby incorporated by reference. According to this method, the ore issubjected to an oxidative bioleach to oxidize the sulfide component ofthe ore and liberate the precious metal values. The ore is theninoculated with a bacterial consortium in the presence of nutrientstherefor to promote the growth of the bacterial consortium, thebacterial consortium being characterized by the property of deactivatingthe preg-robbing propensity of the carbonaceous matter in the ore. Inother words, the bacterial consortium functions as a biological blankingagent. Following treatment with the microbial consortium capable ofdeactivating the precious-metal-adsorbing carbon, the ore is thenleached with an appropriate lixiviant to cause the dissolution of theprecious metal in the ore.

Oxidation of refractory sulfide ores using microorganisms, or as it isoften referred to, biooxidation, can be accomplished in a mill processor a heap process. Compared to pressure oxidation and roasting,biooxidation processes are simpler to operate, require less capital, andhave lower operating costs. Indeed, biooxidation is often chosen as theprocess for oxidizing sulfide minerals in refractory sulfide oresbecause it is economically favored over other means to oxidize the ore.However, because of the slower oxidation rates associated withmicroorganisms when compared to chemical and mechanical means to oxidizesulfide refractory ores, biooxidation is often the limiting step in themining process.

One mill type biooxidation process involves comminution of the orefollowed by treating a slurry of the ore in a stirred bioreactor wheremicroorganisms can use the finely ground sulfides as an energy source.Such a mill process was used on a commercial scale at the Tonkin Springsmine. However, the mining industry has generally considered the TonkinSprings biooxidation operation a failure. A second mill typebiooxidation process involves separating the precious metal bearingsulfides from the ore using conventional sulfide concentratingtechnologies, such as floatation, and then oxidizing the sulfides in astirred bioreactor to alleviate their refractory nature. Commercialoperations of this type are in use in Africa, South America andAustralia.

Biooxidation in a heap process typically entails forming a heap withcrushed refractory sulfide ore particles and then inoculating the heapwith a microorganism capable of biooxidizing the sulfide minerals in theore. After biooxidation has come to a desired end point, the heap isdrained and washed out by repeated flushing. The liberated preciousmetal values are then ready to be leached with a suitable lixiviant.

Typically precious metal containing ores are leached with cyanidebecause it is the most efficient leachant or lixiviant for the recoveryof the precious metal values from the ore. However, if cyanide is usedas the lixiviant, the heap must first be neutralized.

Because biooxidation occurs at a low, acidic pH while cyanide processingmust occur at a high, basic pH, heap biooxidation followed byconventional cyanide processing is inherently a two step process. As aresult, processing options utilizing heap biooxidation must separate thetwo steps of the process. This is conventionally done by separating thesteps temporally. For example, in a heap biooxidation process of arefractory sulfide gold ore, the heap is first biooxidized and thenrinsed, neutralized and treated with cyanide. To accomplish thiseconomically and practically, most heap biooxidation operations use apermanent heap pad in one of several ore on-ore off configurations.

Of the various biooxidation processes available, heap biooxidation hasthe lowest operating and capital costs. This makes heap biooxidationprocesses particularly applicable to low grade or waste type ores, thatis ores having a gold (or an equivalent precious metal value)concentration of less than about 0.07 oz/ton. Heap biooxidation,however, has very slow kinetics compared to mill biooxidation processes.Heap biooxidation typically requires many months in order to oxidize thesulfide minerals in the ore sufficiently to permit the gold or otherprecious metal values to be recovered in sufficient quantities bysubsequent cyanide leaching for the process to be considered economical.Heap biooxidation operations, therefore, become limited by the length oftime required for sufficient biooxidation to occur to permit theeconomical recovery of gold. The longer the time required forbiooxidation the larger the permanent pad facilities and the larger thenecessary capital investment. At mine sites where the amount of landsuitable for heap pad construction is limited, the size of the permanentpad can become a limiting factor in the amount of ore processed at themine and thus the profitability of the mine. In such circumstances, ratelimiting conditions of the biooxidation process become even moreimportant.

The rate limiting conditions of the heap biooxidation process includeinoculant access, nutrient access, air or oxygen access, toxins buildup, and carbon dioxide access, which are required to make the processmore efficient and thus an attractive treatment option. Moreover, forbiooxidation, the induction times concerning biooxidants, the growthcycles, the biocide activities, viability of the bacteria and the likeare important considerations because the variables such asaccessibility, particle size, settling, compaction and the like areeconomically irreversible once a heap has been constructed. This isbecause heaps cannot be repaired once formed, except on a limited basis.

Ores that have a high clay and/or fines content are especiallyproblematic when processing in a heap leaching or heap biooxidationprocess. The reason for this is that the clay and/or fines can migratethrough the heap and plug channels of air and liquid flow, resulting inpuddling; channelling; nutrient-, carbon dioxide-, or oxygen-starving;uneven biooxidant distribution, and the like. As a result, large areasof the heap may be blinded off and ineffectively leached. This is acommon problem in cyanide leaching and has lead to processes of particleagglomeration with cement for high pH cyanide leaching and with polymersfor low pH bioleaching. Polymer agglomerate aids may also be used inhigh pH environments, which are customarily used for leaching theprecious metals, following oxidative bioleaching of the iron sulfides inthe ore.

Biooxidation of refractory sulfide ores is especially sensitive toblocked percolation channels by loose clay and fine material because thebacteria need large amounts of air or oxygen to grow and biooxidize theiron sulfide particles in the ore. Air flow is also important todissipate heat generated by the exothermic biooxidation reaction,because excessive heat can kill the growing bacteria in a large, poorlyventilated heap.

The methods disclosed in U.S. Pat. No. 5,246,486, issued Sep. 21, 1993,and U.S. Pat. No. 5,431,717, issued on Jul. 11, 1995 to William Kohr,both of which are hereby incorporated by reference, are directed toincreasing the efficiency of the heap biooxidation process by ensuringgood fluid flow (both gas and liquid) throughout the heap.

Ores that are low in sulfide or pyrite, or ores that are high in acidconsuming materials such as calcium carbonate or other carbonates, mayalso be problematic when processing in a heap biooxidation. The reasonfor this is that the acid generated by these low pyrite ores isinsufficient to maintain a low pH and high iron concentration needed forbacteria growth.

Solution inventory and solution management also pose important ratelimiting considerations for heap biooxidation processes. The solutiondrained from the biooxidation heap will be acidic and contain bacteriaand ferric ions. Therefore, this solution can be used advantageously inthe agglomeration of new ore or by recycling it back to the top of theheap. However, toxic and inhibitory materials can build up in this offsolution. For example, ferric ions, which are generally a useful aid inpyrite leaching, are inhibitory to bacteria growth when theirconcentration exceeds about 30 g/L. Other metals that retard thebiooxidation process can also build-up in this solution. Such metalsthat are often found in refractory sulfide ores include arsenic,antimony, cadmium, lead, mercury, and molybdenum. Other toxic metals,biooxidation byproducts, dissolved salts and bacterially producedmaterial can also be inhibitory to the biooxidation rate. When theseinhibitory materials build up in the off solution to a sufficient level,recycling of the off solution becomes detrimental to the rate at whichthe biooxidation process proceeds. Indeed, continued recycling of an offsolution having a sufficient build-up of inhibitory materials will stopthe biooxidation process altogether.

The method disclosed in U.S. patent application Ser. No. 08/329,002,filed Oct. 25, 1994, by Kohr, et al., hereby incorporated by reference,teaches a method of treating the bioleachate off solution to minimizethe build-up of inhibitory materials. As a result, when the bioleachateoff solution is recycled to the top of the heap, the biooxidation ratewithin the heap is not slowed, or it will be slowed to a lesser degreethan if the off solution were recycled without treatment.

While the above methods have improved the rate at which heapbiooxidation processes proceed, heap biooxidation still takes muchlonger than a mill biooxidation process such as a stirred bioreactor.Yet, as pointed out above, with low grade refractory sulfide ores, astirred bioreactor is not a viable alternative due to its high initialcapital cost and high operating costs. A need exists, therefore, for aheap bioleaching technique that can be used to biooxidize precious metalbearing refractory sulfide ores and which provides improved air andfluid flow within the heap. In addition, a need exists for a heapbioleaching process in which ores that are low in sulfide minerals, orores that are high in acid consuming materials such as calciumcarbonate, may be processed.

A need also exists for a biooxidation process that can be used toliberate occluded precious metals in concentrates of refractory sulfideminerals. Mill processes that are currently used for oxidizing suchconcentrates include bioleaching in a stirred bioreactor, pressureoxidation in an autoclave, and roasting. These mill processes oxidizethe sulfide minerals in the concentrate relatively quickly, therebyliberating the entrapped precious metals. However, unless theconcentrate has a high concentration of gold, it does not economicallyjustify the capital expense or high operating costs associated withthese processes. And, while a mill bioleaching process is the leastexpensive mill process in terms of both the initial capital costs andits operating costs, it still does not justify processing concentrateshaving less than about 0.5 oz. of gold per ton of concentrate, whichtypically requires an ore having a concentration greater than about 0.07oz. of gold per ton. Therefore, a need also exists for a process thatcan be used to biooxidize concentrates of precious metal bearingrefractory sulfide minerals at a rate comparable to a stirred tankbioreactor, but that has capital and operating costs more comparable tothat of a heap bioleaching process.

In addition to concentrates of precious metal bearing sulfide minerals,there are many sulfide ores that contain metal sulfide minerals that canpotentially be treated using a biooxidation process. For example, manycopper ores contain copper sulfide minerals. Other examples include zincores, nickel ores, and uranium ores. Biooxidation could be used to causethe dissolution of metal values such as copper, zinc, nickel and uraniumfrom concentrates of these ores. The dissolved metal values could thenbe recovered using known techniques such as solvent extraction, ironcementation, and precipitation. However, due to the sheer volume of thesulfide concentrate formed from sulfide ores, a stirred bioreactor wouldbe prohibitively expensive, and standard heap operations would simplytake too long to make it economically feasible to recover the desiredmetal values. A need also exists, therefore, for an economical processfor biooxidizing concentrates of metal sulfide minerals produced fromsulfide ores to thereby cause the dissolution of the metal values sothat they may be subsequently recovered from the bioleachate solution.

Therefore, while a need exists for a method of biooxidation that can beused to process sulfide concentrates from refractory sulfide ores at arate which is much faster than that of existing heap biooxidationprocesses, yet which has initial capital costs and operating costs lessthan that of a stirred bioreactor, this need has gone unfulfilled.Further, while a need has also existed for a method of biooxidation thatcan be used to economically process sulfide concentrates of metalsulfide type ores, this need has also gone unfulfilled.

SUMMARY

The present invention is directed to the biotreatment of solid materialsin a nonstirred bioreactor. To this end, in a first aspect of thepresent invention, a method of biotreating a solid material to remove anundesired compound using a nonstirred surface bioreactor is provided.According to the method the surface of a plurality of coarse substratesis coated with a solid material to be biotreated to form a plurality ofcoated coarse substrates. A nonstirred surface reactor is then formed bystacking the plurality of coated coarse substrates into a heap orplacing the plurality of coated coarse substrates into a tank so thatthe void volume of the reactor is greater than or equal to about 25%.The reactor is inoculated with a microorganism capable of degrading theundesired compound in the solid material, and the solid material is thenbiotreated in the surface bioreactor until the undesired compound in thesolid material is degraded to a desired concentration. To ensureadequate void volume in the bioreactor, the coarse substrates preferablyhave a particle size greater than about 0.3 cm and the solid material tobe biotreated preferably has a particle size less than about 250 μm. Thethickness of the solid material coating on the plurality of coarsesubstrates is preferably less than about 1 mm to ensure that themicroorganism being used in the biotreatment have adequate access to allof the solid material being biotreated. Thicker coatings will increasethe capacity of the bioreactor, but the rate at which the biotreatmentprocess advances will be slowed due to the limited access of themicroorganism being used to the underlying particles of solid material.To make full use of the capacity of the bioreactor while ensuringadequate microorganism access, the thickness of the solid materialcoating should be greater than about 0.5 mm and less than about 1 mm.For enhanced air and liquid access, the void volume of the bioreactorcan be set to greater than or equal to about 35%. This will greatlyimprove the rate at which the biotreatment process proceeds.

A variety of materials can be used for the coarse substrates, includingrock, gravel, lava rock, barren rock containing carbonate minerals,brick, cinder block, slag, and plastic.

The process according to the first aspect of the invention is useful formany different biotreatment processes, including the bioremediation ofcontaminated soils, the desulfurization of coal, and the biooxidation ofrefractory sulfide ores. In bioremediation applications, the undesiredcompound is typically an organic compound. In coal desulfurization andrefractory sulfide ore biooxidation applications, the undesired compoundis sulfide minerals.

In a second aspect of the present invention, a method of biooxidizingsulfide minerals using a nonstirred surface bioreactor to liberate metalvalues of interest is provided. The method comprises obtaining aconcentrate of metal sulfide particles from the sulfide ore body to bebiooxidized and then coating the concentrate of metal sulfide particlesonto a plurality of substrates, such as coarse ore particles, lava rock,gravel, or rock containing carbonate minerals as a source of CO₂ for thebacteria. After the metal sulfide particles are coated or spread ontothe plurality of substrates, a heap is formed with the coated substratesor the coated substrates are placed within a tank. The metal sulfideparticles on the surface of the plurality of coated substrates are thenbiooxidized to liberate the metal values of interest.

Depending on the particular ore deposit being mined, the sulfide mineralconcentrates used in this invention may comprise sulfide concentratesfrom precious metal bearing refractory sulfide ores or they may comprisesulfide concentrates from base metal sulfide type ores, such aschalcopyrite, millerite or sphalorite. The distinction being that in theformer, the metal of interest is a precious metal occluded within thesulfide minerals, and in the latter, the metal to be recovered is a basemetal such as copper, nickel, or zinc and is present as a metal sulfidein the sulfide concentrate.

In a third aspect of the present invention, a method of recoveringprecious metal values from precious metal bearing refractory sulfide oreusing a nonstirred surface bioreactor is provided. The method accordingto this aspect of the invention comprises the steps of producing aconcentrate of metal sulfide particles from the refractory sulfide ore,coating the surface of a plurality of substrates with the concentrate ofmetal sulfide particles, forming a heap using the plurality of coatedsubstrates, biooxidizing the metal sulfide particles on the surface ofthe plurality of substrates, contacting the biooxidized metal sulfideparticles with a precious metal lixiviant to thereby dissolve preciousmetal values from the biooxidized metal sulfide particles, andrecovering precious metal values from the lixiviant.

According to a fourth aspect of the present invention, a method ofrecovering precious metal values from precious metal bearing refractorysulfide ore using a nonstirred surface bioreactor is provided. Themethod according to this aspect of the invention comprises the steps ofproducing a concentrate of metal sulfide particles from a precious metalbearing refractory sulfide ore, coating the surface of a plurality ofcoarse substrates with the concentrate of metal sulfide particles,placing the plurality of coated substrates in a tank, biooxidizing themetal sulfide particles on the surface of the plurality of coarsesubstrates, contacting the biooxidized metal sulfide particles with aprecious metal lixiviant to thereby dissolve precious metal values fromthe biooxidized metal sulfide particles, and recovering precious metalvalues from the lixiviant.

According to a fifth aspect of the present invention, a method forrecovering metal values from a sulfide mineral ore using a nonstirredsurface bioreactor is provided. The method according to this aspect ofthe invention comprises the steps of: producing a concentrate of metalsulfide particles from the sulfide mineral ore, coating the surface of aplurality of coarse substrates with the concentrate of metal sulfideparticles, forming a heap with the plurality of coated substrates orplacing the coated substrates into a tank, biooxidizing the metalsulfide particles on the surface of the plurality of coarse substratesto thereby cause the production of a bioleachate off solution,recovering the desired metal values from the bioleachate off solution.Ores of particular interest that can be processed using this processinclude sulfide ores of copper, zinc, nickel, molybdenum, cobalt, anduranium.

The above and other objects, features and advantages will becomeapparent to those skilled in the art from the following description ofthe preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a process flow chart according toone embodiment of the present invention;

FIG. 2 is a cross sectional view of a refractory sulfide ore substratecoated with a concentrate of metal sulfide particles in accordance withthe present invention;

FIG. 3 is a schematic illustration of a process flow chart according toanother embodiment of the present invention;

FIG. 4 is a schematic illustration of a process flow chart according toyet another embodiment of the present invention;

FIG. 5 is a schematic illustration of a process flow chart according toyet another embodiment of the present invention;

FIG. 6 is a graph illustrating the percent of iron oxidation versus timefor a whole ore compared to a process according to the presentinvention;

FIG. 7 is a graph comparing the average daily biooxidation rate of awhole ore against that of a process according to the present invention;

FIG. 8 is a graph illustrating the percentage of biooxidation foranother process according to the present invention;

FIG. 9 is a graph illustrating the average daily rate of biooxidationfor the process corresponding to FIG. 8; and

FIG. 10 is a graph illustrating the percentage of biooxidation as afunction of time for a pyrite concentrate coated on a barren rocksupport and the same pyrite concentrate coated on a refractory sulfideore support that contains a high concentration of mineral carbonate.

DETAILED DESCRIPTION

A first embodiment of the invention is now described in which a solidmaterial is biotreated in a nonstirred surface bioreactor in order toremove an undesired compound. According to the first embodiment, thesurface of a plurality of coarse substrates having a particle sizegreater than about 0.3 cm is coated with the solid material to bebiotreated to form a plurality of coated coarse substrates. The solidmaterial to be biotreated has a particle size of less than about 250 μmso that it forms a fairly uniform coating on the coarse substrates. Anonstirred surface reactor is then formed by stacking the plurality ofcoated coarse substrates into a heap or placing the plurality of coatedcoarse substrates into a tank so that the void volume of the reactor isgreater than or equal to about 25%. The reactor is inoculated with amicroorganism capable of degrading the undesired compound in the solidmaterial, and the solid material is then biotreated in the surfacebioreactor until the undesired compound in the solid material isdegraded to a desired concentration.

The biotreatment process can be used in the bioremediation ofcontaminated soils, the desulfurization of coal, and the biooxidation ofrefractory sulfide ores to name a few. In bioremediation applications,the solid material is typically soil and the undesired compound istypically an organic compound within the soil. The present invention,therefore, has application at many of the existing superfund sites. Apartial list of the organic contaminants which can be removed from soilusing the present invention include: waste oil, grease, jet fuel, dieselfuel, crude oil, benzene, toluene, ethylbenzene, xylene, polyaromatichydrocarbons (PAH), polynuclear aromatics (PNAs), pentachlorophenol(PCP), polychlorinated biphenyls (PCBs), creosote, pesticides,2,4,6,-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine(RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX),N-methyl-N-2,4,6-tetranitroaniline, and nitrocellulose (NC).

If, on the other hand, the present invention is used to desulfurizecoal, the solid material will be comprised of coal particles and theundesired compound will be the sulfide mineral particles containedwithin the coal particles. In refractory sulfide ore biooxidationapplications, the solid material will typically be ground ore or asulfide concentrate produced from the ore and the undesired compoundwill be the metal sulfide particles within the ore or concentrate.

In some instances it may be beneficial to form a concentrate byflotation or by other means where by the fraction of the solid materialto be biotreated is concentrated in a smaller weight fraction. Thisconcentrate, if it contains the majority of the undesired metal sulfidesor toxins, for example, can be processed more cost effectively than theentire material.

As those skilled in the art will appreciate from the foregoing and theensuing description, the process according to the present invention hasbroad applicability in that it can be used to biotreat any solidmaterial that contains an undesired compound which is susceptible tobiodegradation or biooxidation by a microorganism or the enzymesproduced by a microorganism.

The purpose of the coarse substrates is to provide a support with arelatively large surface area upon which the solid material to bebiotreated can reside during the biotreatment process. Therefore, when alarge number of coated coarse substrates are stacked in a heap or placedin a tank, a nonstirred surface reactor is formed that has a very largeactive surface area per cubic meter of reactor space. Although the exactsurface area of the reactor per cubic meter of reactor space will dependon the particular size of the coarse substrates employed, it should beat least 100 square meters per cubic meter of reactor and will typicallybe 500 square meters or more per cubic meter of reactor space.Furthermore, by using coarse substrates that have a particle sizegreater than about 0.3 cm and restricting the particle size of the solidmaterial to be biotreated to less than about 250 μm, the reactor will beensured adequate void volume to permit air and nutrients to access allparts of the reactor during the biotreatment process. In this regard,the void volume of the reactor should be at least about 25%. Such a voidvolume will also ensure adequate heat dissipation within the heap. Forenhanced air and liquid access and heat dissipation, the void volume ofthe bioreactor can be set to greater than or equal to about 35%. Thiswill greatly improve the rate at which the biotreatment processproceeds.

While using larger coarse substrates will increase the void volume inthe reactor and thus improve air and nutrient access, in addition toheat dissipation, throughout the entire reactor, the use of largersubstrates reduces the loading capacity of the bioreactor. A goodcompromise between ensuring adequate void volume and ensuring adequatereactor capacity can be achieved by using coarse substrates having anominal particle size that is greater than about 0.6 cm and less thanabout 2.54 cm.

A variety of materials can be used for the coarse substrates, includingrock, gravel, lava rock, barren rock containing carbonate minerals,brick, cinder block, slag, and plastic. Lava rock is particularlypreferred because of its rough, nonuniform surface, thus increasing itssurface area for a given particle size substrate and improving theintegrity of the coating of solid material which is applied to it.Coarse barren rock containing carbonate minerals is advantageous if thebiotreatment process is acidic because the acid will react with thecarbonate minerals to slowly cause the release of carbon dioxide, whichautotrophic microorganisms can use as a source of carbon to carry outmetabolic synthesis. The carbon dioxide production can thus be used topromote microorganism growth in the reactor.

When a refractory sulfide ore or sulfide concentrate is beingbiooxidized to reduce the sulfide mineral content therein, coarse oreparticles can be used as the coarse substrates. Similarly, if theprocess is being used to desulfurize coal, coarse coal particles can beused as the coarse substrates. In both cases, the substrate may benefitfrom the biooxidation process carried out on its surface.

While the coarse substrates have been defined as having a particle sizeof greater than about 0.3 cm, it is recognized and contemplated thatsome of the coarse substrate material may actually be smaller than this.As those skilled in the art will recognize that if the coarse substratesare produced by crushing lager material to the desired size range, thecrushed material will have a certain size distribution. And, even if thematerial is screened to exclude material less than about 0.3 cm, somematerial having a particle size less than the 0.3 cm target minimum willstill be present in the coarse substrates due to inherent inefficienciesin the screening process and due to particle attrition during handling.Thus, by greater than about 0.3 cm it is intended that substantially allof the coarse substrates are above this size so that the void volume ofthe reactor remains above at least about 25% during formation of thereactor and throughout its operation. Preferably the amount of coarsesubstrates below the 0.3 cm cutoff is less than 5% by weight.

In general, the solid material to be biotreated should be much smallerthan the coarse substrate onto which it is coated. This material shouldbe ground to a small enough size to allow the microorganism employed inthe biotreatment to have access to all the material so that theundesired compound can be biooxidized or biodegraded in a time that isgenerally larger than a stirred tank process, but shorter than a heapprocess of the whole material. This time will generally be between 14days and 90 days, depending on the undesired compound and the rate ofits biodegradation or biooxidation.

The maximum solid material particle size has been set at about 250 μm sothat the solid material will form a relatively uniform coating on thecoarse substrates during the coating process, rather than formingagglomerates between themselves. Furthermore, particles larger than 250μm may not adhere to the surface of the coarse substrates very wellwithout the use of a binder.

It is desirable to form a relatively uniform coating of the fineparticles on the coarse substrates during the coating process becausethis will maximize the integrity of the coating and the surface area ofthe solid material exposed to the active microorganism which is added tothe bioreactor. If agglomerates of the solid material are formed duringthe coating process, the particles of solid material which are in theinterior of the agglomerate will be blocked from the action of themicroorganism and thus the amount of biological treatment they willreceive will be reduced or nonexistent. Further, the agglomerates arenot as structurally sound as the coated substrates and are likely tobreak apart during the stacking process used to form the reactor orduring biotreatment, potentially leading to the formation of blockageswithin the reactor, which could blind off portions of the reactor fromthe biological treatment.

Typically as the particle size of the solid material to be biotreateddecreases, the biotreatment process will proceed faster and more solidmaterial can be loaded onto the coarse substrates. Smaller particlesizes will also tend to stick better to the surface of the coarsesubstrates. If the particle size of the solid material to be treated isless than about 25 μm, however, excessive dust problems could beencountered during handling and some clumping may be experienced duringthe coating process.

Preferably the particle size of the solid material to be treated has anominal particle size which is greater than about 75 μm and less thanabout 106 μm. Particles in this size range will adhere well to thecoarse substrates, and the incremental improvements which can beachieved in the rate of the biotreatment process with finer particlesizes are rarely justified by the added grinding costs of producingthem.

The coated substrates can be produced by adding the coarse substratesand solid material to a rotating drum in appropriate quantities.Preferably the coarse substrates are dry and the solid material is in ahigh pulp density slurry so that it will stick to the coarse substratesas the slurry coats the coarse substrates. Alternatively, both thecoarse substrates and solid material can be dry when added to therotating drum and water sprayed into the drum to promote adhesion of thesolid material to the coarse substrates. In forming the coatedsubstrates, it is desirable to maintain the moisture content of thesolid material within the range of 5 to 30 weight % to promote properadhesion between the solid material and coarse substrates.

As those skilled in the art will recognize many other techniques canalso be used to coat the coarse substrates. For example, the solidmaterial to be biotreated can be sprayed in a high pulp density slurryform onto the coarse substrates as the plurality of coarse substratesare being stacked to form the reactor.

If the solid material to be biotreated is applied as a slurry,adjustments can be made to the material to optimize the biotreatmentprocess. For example, the pH can be adjusted to the optimum pH range forthe microorganism that is to be used to break down the undesiredcompound. If nutrients, amendments, or inoculants are needed, they canalso be added at this time. In some cases it may be advantageous tostart the bioprocess in a tank prior to application of the particles ofsolid material to the coarse substrates.

The integrity of the coated coarse substrates should be sufficientenough to prevent a large number of blockages from forming in the flowchannels of the reactor while the particles of solid material on thesurface of the coated substrates are being biotreated. Such blockageswill decrease oxygen flow and microorganism migration within thebioreactor and thereby reduce the rate of the biotreatment process. Ofcourse, the larger the coarse substrates are in relation to the particlesize of the solid material, the less likely such blockages will formbecause the solid material will be much smaller than the intersticesbetween the coarse substrates. The integrity of the coated substratesshould also be sufficient enough to prevent excessive amounts of thesolid material from washing from the bioreactor during the biotreatmentprocess.

Although the surface tension of water should hold the particles of solidmaterial to the surface of the coarse substrates in most instances, ifit is found that the particles of solid material are washing from thebioreactor in excessive concentrations or that blockages are forming inthe bioreactor due to degradation of the coating, a binding agent can beused to improve the integrity of the coating. However, binding agentsmay interfere with the access of the biotreatment microorganism to someof the solid material to be biotreated, thus increasing the timenecessary for the biotreatment process to reach the desired end point.

The thickness of the solid material coating on the plurality of coarsesubstrates is preferably less than about 1 mm to ensure that themicroorganism being used in the biotreatment have adequate access to allof the solid material being biotreated. Thicker coatings will increasethe capacity of the bioreactor, but the rate at which the biotreatmentprocess advances will be slowed due to the limited access of themicroorganism being used to the underlying particles of solid material.To make full use of the capacity of the bioreactor while ensuringadequate microorganism access, the thickness of the solid materialcoating should be greater than about 0.5 mm and less than about 1 mm.When a rock or brick substrate is being used, this will translate into asolid material loading of approximately 10 to 30 percent by weight.

The nonstirred surface reactor is formed by stacking a plurality of thecoated substrates in a heap or in a tank. Conveyor stacking willminimize compaction of the coated substrates within the reactor.However, other means of stacking may be employed.

Preferably the reactor is inoculated with the microorganism(s) which isto be used in the biotreatment process while the plurality of coatedsubstrates are being stacked to form the nonstirred surface reactor orimmediately after formation of the reactor. Alternatively, if themicroorganism(s) to be employed in the biotreatment process functionbest in a particular pH range, the pH of the reactor can be adjustedprior to inoculation as is well known in the art.

The microorganisms which are useful in the present biotreatment processare the same microorganisms that have traditionally been used to degradea particular undesired compound in existing biodegradation andbiooxidation processes. For example, acidophilic, autotrophic bacteriasuch as Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, andSulfolobus, can be used to biooxidize sulfide minerals in coaldesulfurization or refractory sulfide ore biooxidation applications.Other bacteria that are useful in these applications are well within theordinary skill of those in the art. Similarly, with respect to soilremediation applications, the microorganism(s) which should be employedare the same as those currently employed in present bioremediationprocesses such as composting, landfarming, slurry biodegradation, andheap pile bioremediation. Those having ordinary skill in the art will bereadily able to determine which microorganism(s) are applicable for thevarious undesired compounds which may be removed from the solid materialusing the process according to the present invention.

Once the reactor is inoculated with an appropriate microorganism, theconditions such as pH, temperature, nutrient supply, and moisturecontent within the reactor should be monitored and maintained throughoutthe biotreatment so as to promote the growth of the microorganism to thefullest extent possible. As the microorganism grows throughout thereactor, the reactor is transformed into a bioreactor having a verylarge surface area that will biodegrade or biooxidize the undesiredcompound in a time much shorter than that of traditional static batchbiotreatment processes such as heap bioleaching, composting, andlandfarming.

The reactor can also be provided with perforated air pipes through whichair can be blown or drawn as is well known in the art. Whether air isblown or drawn through the reactor will depend on the specificbioprocess occurring within the reactor, and such a selection is alsowell within the skill of those in the art.

The biotreatment process should be permitted to proceed until theundesired compound in the solid material is degraded to a desiredconcentration. In the case of soil remediation applications, this willtypically be dictated by governmental regulations which define theacceptable level of a particular contaminant. In coal desulfurizationapplications, the amount of residual sulfur which is permitted to remainin the coal will also depend, to a large extent, on environmentalregulations, because when sulfur bearing coal is burned it will producesulfur dioxide as a byproduct. Thus, the amount of sulfur allowed toremain in the coal should be less than that which would violateenvironmental regulations when the coal is burned. This, of course, willdepend to some extent on the equipment employed at the coal fired plantwhere the biotreated coal will be utilized. With respect to thebiooxidation of refractory sulfide ores or concentrates, the amount ofsulfide mineral that is permitted to remain in the ore will be dictatedby the amount that must be biooxidized to achieve economical recoveriesof the desired metal values from the ore or concentrate.

After the undesired compound has been reduced to a desiredconcentration, the bioreactor can be broken down and the biotreatedsolid material separated from the coarse substrates. After separation ofthe biotreated solid material, the coarse substrates can be reused.After one or more uses in the biotreatment process, a film of themicroorganism used in the biotreatment process will develop on thesubstrates. This biofilm will have the advantage of adaptation to anytoxic or inhibitory materials that are present in the solid materialbeing processed. It is therefore best to remove the biotreated solidmaterial in such a way as to not kill or entirely remove the biofilmthat has built up on the coarse substrates. The biofilm is also anefficient way to inoculate the next coating of solid material applied tothe coarse substrates. Finally, the adaptation of the microorganismafter having been through the process many times will also speed up therate at which the microorganism biodegrades or biooxidizes the undesiredcompound in the solid material being processed.

The present invention will now be described in further detail inconnection with a number of possible embodiments that can be employed inthe processing of refractory sulfide ores.

The second embodiment of the present invention is described inconnection with FIGS. 1 and 2. FIG. 1 illustrates a process flow chartfor liberating and recovering precious metal values from precious metalbearing refractory sulfide ores. For purposes of describing the processillustrated in FIG. 1, the sulfide mineral concentrate 22 used in thepresent embodiment is produced from a gold bearing refractory sulfideore. It follows, therefore, that the precious metal recovered in thepresent embodiment is gold. However, as one skilled in the art wouldunderstand, other precious metals, such as platinum and silver, can alsobe liberated and recovered from refractory sulfide ores using theprocess illustrated in FIG. 1. A combination of precious metals can alsobe recovered using the process according to the present embodiment ifthe refractory sulfide ore body used to produce the sulfide mineralconcentrate 22 contains more than one precious metal.

According to the process flow chart shown in FIG. 1, a plurality ofsubstrates 20 and a sulfide mineral concentrate 22 are added to arotating drum 24. Preferably the sulfide mineral concentrate 22 is in aslurry form and the plurality of substrates 20 are dry when added torotating drum 24 to improve the adhesion between the substrates 20 andthe concentrate 22. Optionally, a polymeric binding agent can be addedto rotating drum 24, although it is not necessary. As rotating drum 24rotates, the substrates 20 added to drum 24 are coated with the wetsulfide mineral concentrate 22 to form coated substrates 39. Coatedsubstrates 39 are then stacked to form static heap 26.

By using a slurry of concentrate in the coating process, the need andcost of drying the concentrate after its production is eliminated.Concentrate 22 and the plurality of substrates 20 can, however, be addedto rotating drum 24 in the dry state, in which case after the mixture isadded to drum 26 it is sprayed with water or an aqueous acid solution,preferably containing ferric ions, to cause the concentrate to stick tothe substrates. The benefit of using an aqueous acid solution containingferric ions to bind the concentrate to the surface of the substrates isthat it will begin to chemically oxidize the sulfide mineralconcentrate. Also it is acidic so that it will lower the pH of thecoated substrates 39 in preparation for biooxidation. The disadvantageof using such an acid solution is that it will increase the cost of theequipment used to form the coated substrates 39 because it must bedesigned to be acid resistant.

Sulfide mineral concentrate 22 is comprised of a plurality of fine metalsulfide particles 40 which have finely disseminated gold and possiblyother precious metal values occluded within. Sulfide mineral concentrate22 will also typically contain fine particles of sand or other ganguematerial 42 from the refractory sulfide ore from which concentrate 22 isobtained. As a result, each of the coated substrates 39 will be coatedwith the metal sulfide particles 40 and fines 42 as illustrated in FIG.2.

The integrity of coated substrates 39 should be sufficient enough toprevent a large number of blockages from forming in the flow channelswithin heap 26 while the metal sulfide particles 40 on the surface ofcoated substrates 39 are being biooxidized. Such blockages decreaseoxygen flow and bacteria migration within the heap and thereby reducethe rate of biooxidation.

Because metal sulfide particles 40 are hydrophobic, they will tend tostick to the dry substrates 20 without the use of a binding agent suchas a polymeric agglomeration aid. This assumes, however, that the metalsulfide particles 40 are of an appropriate size. Therefore, ifconcentrate 22 contains an adequate concentration of metal sulfideparticles 40, concentrate 22 will remain sufficiently adhered to coatedsubstrates 39, even without the use of a binding agent, to permit coatedsubstrates 39 to be handled while being stacked on heap 26 or placed intank 45, which is described later in connection with the embodimentillustrated in FIG. 5. Furthermore, coated substrates 39 should retaintheir integrity throughout the biooxidation process. When forming coatedsubstrates 39 without the use of a binding agent, therefore, it isimportant to use a sulfide mineral concentrate which has an adequateconcentration of metal sulfide particles and an appropriate particlesize.

While a polymeric binding agent can be used and would possibly improvethe integrity of the coated substrates 39, the use of such agents willincrease the operating cost of the process.

Several factors need to be taken into consideration when determining theappropriate concentration of metal sulfide particles 40 in concentrate22. First, higher concentrations of metal sulfides are desirable in theconcentrate so that more metal sulfide particles 40 can be processed perunit surface area of substrates 20. This is advantageous in that as theloading of metal sulfide particles increases, the rate of biooxidationin heap 26 will tend to increase. Furthermore, because the preciousmetal values are occluded within the metal sulfide particles 40, higherconcentrations of these particles in concentrate 22 will tend to resultin improved recovery rates for a particular ore body, in addition tolowering the cost of processing the concentrate per ounce of goldproduced.

A second factor that weighs in favor of producing a concentrate 22 thatcontains as much metal sulfides as practicable is that the potential forthe formation of blockages in the flow channels of heap 26 is reduced byminimizing the amount of gangue material 42 in concentrate 22. Thereason being that the fine particles of gangue material 42 are morehydrophilic than the fine metal sulfide particles 40, and, as a result,they tend to adhere to the surface of substrate 20 less tenaciously. Thefine particles of gangue material 42 will, therefore, tend to migratethrough the heap with the added bioleachant maintenance fluids duringbiooxidation, which in turn increases the likelihood that blockages willform in flow channels of heap 26. Accordingly, as the concentration ofmetal sulfide particles approaches 20 weight %, it may be desirable oreven necessary to use a polymeric agglomeration aid to ensure sufficientintegrity of the coated substrates 39 during handling and biooxidation.On the other hand, by using a sulfide mineral concentrate with at leastabout 40 weight % metal sulfide particles, coated substrates 39 can bereadily formed without the use of a polymeric agglomeration aid and ahigh degree of loading of metal sulfide particles 40 per unit surfacearea is achieved.

At least two factors militate against using a sulfide mineralconcentrate 22 having a very high concentration of metal sulfideparticles 40. First, the cost of producing concentrate 22 is typicallyproportional to its concentration of metal sulfide particles. Thus, asthe concentration of metal sulfide particles 40 in concentrate 22increases, the cost of producing concentrate 22 will likewise increase.The added cost of producing very high grades of concentrate 22 may notbe offset by the incremental improvement in metal sulfides loading orintegrity of the coated substrates 39. Second, as the grade ofconcentrate increases, the amount of metal sulfide particles 40 thatremain with the tail fraction of the refractory sulfide ore willincrease. Because these metal sulfide particles contain occludedprecious metal values, any metal sulfide particles 40 that remain in theore tail will decrease the total recovery rate for the process.

Taking the above factors into consideration, sulfide mineral concentrate22 should contain at least 20 weight % metal sulfides to ensure adequatehandling characteristics and integrity during biooxidation. Preferably,however, the concentrate will contain at least about 40 weight % metalsulfides, and more preferably at least about 70 weight %. Typically,concentrate 22 will contain between about 40 to 80 weight % metalsulfides.

In general, as the particle size of the sulfide mineral concentrate 20decreases, the faster the biooxidation process will proceed. Smallerparticle sizes also tend to result in improved concentrate grades. Thisis because it is typically easier to separate the metal sulfideparticles 40 from the bulk of gangue material as the particle size ofthe ore is decreased. Sulfide mineral concentrate 22, therefore,preferably has a particle size of less than about 250 μm. Particleslarger than 250 μm may not adhere to substrates 20 very well without theuse of a binding agent. In addition, unless the refractory sulfide orefrom which concentrate 22 is produced is ground to at least 100% passing250 μm, it is difficult obtain a good separation of the metal sulfideparticles 40 from the bulk of gangue material during concentration. Thisis especially true if flotation is used to form concentrate 22, becauseparticles larger than 250 μm do not float very well. On the other hand,if the particle size of concentrate 22 is less than about 38 μm to 25μm, the concentrate particles will tend to clump together during thecoating process rather than form a relatively uniform coating on coatedsubstrate 39. These clumps of concentrate can block air flow andbacteria migration during biooxidation, thereby reducing the rate ofbiooxidation in the heap.

Preferably the particle size of concentrate 22 is about 100% passing 106μm to 75 μm. Particles in this size range adhere well to substrates 20,and the incremental improvements which can be achieved in the rate ofbiooxidation and the concentrate grade with finer particle sizes arerarely justified by the added grinding costs of producing them.

Sulfide mineral concentrate 22 can be produced from any precious metalbearing refractory sulfide ore body being mined using techniques wellknown in the art and thus need not be explained in detail here. Theproduction of concentrate 22, however, will typically include thecrushing and grinding of the refractory sulfide ore to an appropriateparticle size followed by one or more gravity separations or one or moresulfide flotations.

Some potential refractory sulfide ore bodies may already be ofsufficient grade such that further concentration is not required. Suchore bodies may include tailings or waste heaps at existing mines. Whenthese types of ores are processed, the sulfide mineral concentrate needonly be transported to the location of the biooxidation facility andpossibly some additional comminution to achieve the desired particlesize.

With respect to gold concentration, the process according to the presentembodiment can be performed economically even if concentrate 22 containsas little as 5 g Au/metric ton of concentrate (or an equivalent economicvalue of other precious metal values). This number of course will varyto a large extent based on the cost of producing concentrate 22 and theprevailing price of gold. As those skilled in the art will recognize,however, traditional autoclaves or stirred tank bioreactors cannot comeclose to economically processing a sulfide mineral concentrate havingsuch a low concentration of gold.

Many different materials can be used for substrates 20. Preferredsubstrates include coarse refractory sulfide ore particles, lava rock,gravel, and rock which includes a mineral carbonate component. Thepurpose of the substrates 20 is to provide a support with a relativelylarge surface area upon which the concentrate 22 can reside during thebiooxidation process. The surface area of each substrate 20 in effectacts as a small surface bioreactor during biooxidation. Therefore, whena large number of coated substrates 39 are stacked in heap 26 forbiooxidation, a nonstirred surface bioreactor is created that has a verylarge total surface area.

The total surface area of the bioreactor or heap 26 can be increased bydecreasing the particle size of substrates 20, using substrates thathave a rough, nonuniform surface morphology and/or increasing the numberof coated substrates 39 stacked on heap 26. The advantage of increasingthe total surface area of the substrates 20 within heap 26 is that theamount of concentrate 22 that can be loaded on substrates 20 increasesproportionately, which in turn increases the amount of concentrate 22that can be biooxidized in a particular heap 26.

The preferred particle size range for substrates 20 is nominally fromabout +0.62 cm to about −2.5 cm with particles less than about 0.3 cmremoved by screening or other suitable method. However, substrates 20having a particle size down to approximately +600 μm can be used. Whileincreased loading is achieved with smaller substrate particle sizes,increased air flow, fluid flow and heat dissipation is achieved withlarger particle sizes. The nominal +0.62 to −2.5 cm size range providesa good compromise between concentrate loading and ensuring adequate airflow, fluid flow, and heat dissipation.

Substrates 20 are preferably loaded with as much concentrate 22 duringthe coating process as possible to maximize the process throughput. Theamount of concentrate 22 that can be loaded on substrates 20 will dependon particle size and surface morphology of the substrates 20. Coarsesubstrates 20 and sulfide mineral concentrate 22 should, therefore, beadded to rotating drum 24 in sufficient quantities to maximize theamount of sulfide mineral concentrate 22 loaded on each substrate 39while minimizing the formation of agglomerates of the sulfide mineralconcentrate particles. Clumps or agglomerates of the sulfide mineralconcentrate 22 particles may be formed if the particle size of theconcentrate is too fine, as discussed above, or if an excess amount ofthe concentrate is added to drum 24. To ensure adequate loading ofsubstrates 20 while simultaneously avoiding formation of agglomerates ofthe concentrate particles, preferably approximately 10 to 30 weight %concentrate is added to rotating drum 24, which will result in a loadingof approximately 10 to 30 weight % of concentrate 22 on coatedsubstrates 39.

In forming coated substrates 39, it is desirable to maintain themoisture content of concentrate 22 within the range of 5 to 30 weight %.If the moisture content of the concentrate is below 5 weight %, theconcentrate will not adhere properly to the substrates, and if themoisture content exceeds 40 weight %, the concentrate slurry will be toothin to form a thick enough coating on the substrate. This would limitthe amount of concentrate that would adhere to the substrates 20.

Although other means of heap construction may be used, conveyor stackingis preferred. Conveyor stacking minimizes compaction of the coatedsubstrates within the heap. Other means of stacking such as end dumpingwith a dozer or top dumping can lead to regions of reduced fluid flowwithin the heap due to increased compaction and degradation of thecoated substrates.

If desired, heap 26 can be provided with perforated pipes 27 connectedto an air supply source (not shown) in order to increase the air flowwithin the heap. Increasing the air flow within heap 26 will increasethe rate of biooxidation and improve the rate at which heat isdissipated from the heap. Furthermore, because of the large air andfluid flow channels between the coated substrates 39, the air supplysource connected to perforated pipes 27 can be a low cost blower ratherthan a more expensive compressor.

Heap 26 is preferably inoculated with a bacteria capable of biooxidizingmetal sulfide particles 40 while coated substrates 39 are being stackedon to heap 26 or immediately after formation of heap 26 or after the pHof heap 26 has been lowered to below 2.5. The following bacteria may beused in the practice of the present invention:

-   -   Thiobacillus ferrooxidans; Thiobacillus thiooxidans;        Thiobacillus organoparus; Thiobacillus acidophilus;        Leptospirillum ferrooxidans; Sulfobacillus thermosulfidooxidans;        Sulfolobus acidocaldarius; Sulfolobus BC; Sulfolobus        solfataricus and Acidianus brierleyi and the like.

These bacteria are all available from the American Type CultureCollection or like culture collections. Whether one or more of the abovebacteria and the particular bacteria selected for use in the presentprocess will depend on factors such as the type of ore being processedand the expected temperatures in heap 26 during biooxidation. Theseselection criteria, however, are well within the skill of those in theart and need not be described in detail here. The most common andpreferred bacteria for biooxidation is Thiobacillus ferrooxidans.

During the biooxidation of the metal sulfide particles 40 coated on thesurface of the coated substrates 39, additional inoculant and microbialnutrient solutions can be supplied through a sprinkler system 28.Additions of these bioleachant maintenance solutions will typically bemade in response to certain performance indicators used to monitor theprogress of the biooxidation process.

The rate of biooxidation is preferably monitored throughout thebiooxidation process based on selected performance indicators such asthe solubilization rate of arsenic, iron, or sulfur, or the oxidationrate of sulfides which can be calculated therefrom. Other biooxidationperformance indicators that may be used include measuring pH, titratableacidity, and solution Eh. Preferably the bioleachate off solution thatpercolates through the heap is collected at drain 29 and recycled to thetop of heap 26. This minimizes the amount of fresh water required by thebiooxidation process. And, because the bioleachate off solution will beacidic and contain a high concentration of ferric ions, itsreapplication to the top of heap 26 is advantageous to the biooxidationprocess. However, the effluent solution generated early in thebiooxidation process will also contain significant concentrations ofbase and heavy metals, including components that lead to microbialinhibition. As the inhibitory materials build-up in the bioleachate offsolution, the biooxidation process is retarded. Indeed, continuedrecycling of an off solution without treatment can lead to a build-up ofinhibitory materials sufficient to stop the biooxidation processaltogether.

To minimize the build-up of inhibitory materials and thus their effecton the biooxidation process, the off solution can be treated in acidcircuit 30 prior to recycling to remove the inhibitory materials whentheir concentration becomes excessive. One method of conditioning thebioleachate off solution before recycling comprises raising its pH above5, removing any precipitate that forms and then lowering its pH to a pHappropriate for biooxidation using an untreated portion of the offsolution or other acid solution. Such a conditioning process isdisclosed in U.S. patent application Ser. No. 08/547,894, filed Oct. 25,1995 by Kohr et al, which is hereby incorporated by reference.

The bioleachate off solution will tend to be very acidic in the presentinvention. This is because a concentrate having a relatively highconcentration of metal sulfide minerals is being biooxidized rather thanan entire ore. As a result, the biooxidation process according to thepresent invention will tend to produce large amounts of excess acid.That is the process will produce more acid than can be practicallyrecycled to the top of heap 26. This excess acid must be disposed of orused for other purposes. One possible use for the excess acid is in acopper oxide ore leaching process because sulfuric acid is an effectivelixiviant for copper oxide ores. However, the sulfuric acid solutionproduced as a byproduct of the present process will also typicallycontain a high concentration of ferric ions. This also makes it aneffective lixiviant for some copper sulfide ores such as chalcocite. Theferric ion in the acid solution chemically oxidizes the copper sulfideminerals to cause their dissolution. Thus, the excess acid from thepresent process can be beneficially used in a copper leaching operationto avoid the neutralization costs associated with disposal whilesimultaneously reducing the acid costs for the copper leachingoperation.

After the biooxidation reaction has reached an economically defined endpoint, that is after the metal sulfide particles 40 on the surface ofthe coarse substrates 20 are biooxidized to a desired degree, the heapis broken down and the biooxidized concentrate 22 is separated from thecoarse substrates 20. Prior to breaking the heap down, however, the heapwill typically be drained and then washed by repeated flushings withwater. The number of wash cycles employed is typically determined by asuitable marker element such as iron and the pH of the wash effluent.

Separation can be accomplished by placing the coated substrates 39 on ascreen and then spraying the coated substrates with water.Alternatively, the coated substrates can be tumbled in water using atrommel.

Following separation, gold is extracted from the biooxidized concentrate22. This can be accomplished using a number of techniques well known inthe art. Typically, however, the biooxidized concentrate will be leachedwith a lixiviant such as cyanide in a carbon-in-pulp or acarbon-in-leach process. In these processes, the lixiviant dissolves theliberated gold or other precious metal values that are then adsorbedonto activated carbon as is well known in the art.

If cyanide is used as the lixiviant, the concentrate will need to beneutralized prior to leaching. To avoid the need for neutralization,thiourea can be used as the lixiviant to extract the gold from thebiooxidized concentrate. The thiourea extraction process can be improvedby adjusting the Eh of the leach solution using sodium metabisulfite asdisclosed in U.S. Pat. No. 4,561,947, which is incorporated herein byreference. If thiourea is used as the lixiviant, preferably a syntheticresin, rather than activated carbon, is used to adsorb the dissolvedprecious metal values from the lixiviant solution.

After the liberated gold or other precious metal values are extractedfrom the biooxidized concentrate, the biooxidized concentrate is takento a waste or tailings pile 36 and gold is recovered from the carbon orsynthetic resin using techniques well known in the art.

The coarse substrates 20 which have been separated from the biooxidizedconcentrates can be recycled to the rotating drum for a new coating ofsulfide mineral concentrate 22. Substrates 20 can be reused so long asthey retain their mechanical integrity. If coarse refractory sulfide oreparticles are used for substrates 20, they are preferably processed atsome point, preferably after one to three cycles, to recover liberatedgold values.

As illustrated in FIG. 2, coarse refractory sulfide ore substrates 20will contain metal sulfide particles 40 which contain occluded gold andother precious metal values. After one to three cycles through theprocess, many of the metal sulfide particles 40 within the coarse oresubstrates 20 will be partially biooxidized. Rather than continuing torecycle the coarse ore substrates in this situation and allow theliberated gold values to go unclaimed, the coarse ore substrates can beprocessed to recover their gold values. This is preferably accomplishedby grinding the coarse ore substrates in grinding circuit 32 to aparticle size suitable to permit the metal sulfide particles to beseparated from the bulk of the gangue material. A concentrate 22 of themetal sulfide particles 40 from the ground coarse ore substrates is thenproduced in the sulfide concentrator 34. Preferably sulfide concentrator34 is a flotation cell and the biooxidized coarse ore substrates areground to a size appropriate for sulfide flotation and coating onsubstrates 20. The concentrate 22 produced from the ground oresubstrates is then combined with the supply of sulfide mineralconcentrate 22 from which it is coated on a second plurality of coarsesubstrates 20 and added to a new heap 26 for further biooxidation.

The flotation tail from sulfide concentrator 34 should be treated in thegold extraction process along with the biooxidized concentrate 22 fromheap 26. The flotation tail will contain a number of fully and partiallyoxidized metal sulfide particles that did not float. These oxidizedparticles will contain significant gold values, and as much of thesegold values will already be liberated, they can be readily leached fromthe flotation tail using cyanide or thiourea. After lixiviation, theflotation tail is disposed of along with the biooxidized concentratewhich has gone through gold extraction in waste or tailings pile 36.

Refractory sulfide coarse ore substrates 20 that have gone through thebiooxidation process can alternatively be processed simply by grindingfollowed by lixiviation. This process alternative, however, will resultin a lower overall recovery, because many of the metal sulfide particles40 within the coarse ore substrates will not be sufficiently oxidized toliberate their entrapped gold values.

With respect to material selection for substrates 20, there are severaladvantages of using coarse refractory sulfide ore particles.

First, the refractory sulfide ore body being mined will typically haveto go through several crushing and grinding steps before an appropriateparticle size is achieved for producing concentrate 22. As a result,coarse refractory sulfide ore substrates can be removed from anappropriate stage of the crushing process, which makes coarse refractorysulfide ore particles an inexpensive source of substrates 20.

Second, as illustrated in FIG. 2 and discussed above, if coarserefractory sulfide ore is used as the substrate material, it willcontain metal sulfide particles 40. These metal sulfide particles willbe partially biooxidized during the biooxidation process, and, if thecoarse ore particles are recycled through the process several times, themetal sulfide particles 40 will eventually become sufficientlybiooxidized to permit recovery of their precious metal values.

A third advantage, which is somewhat related to the second, is that afraction of the iron sulfide or other metal sulfide particles 40 in therefractory sulfide ore are so fine that they will not float very well inthe concentration process. By using coarse particles of the ore forsubstrates 20, these very fine metal sulfide particles will bechemically oxidized over time by the ferric ion in the bioleachant.Then, when the coarse ore particles are eventually ground and floated toproduce a concentrate of metal sulfide particles, the oxidized finemetal sulfide particles will end up in the flotation tails. Because theflotation tails are leached with cyanide or other lixiviant, theliberated gold values from these very fine sulfide particles will berecovered. On the other hand, if the coarse ore particles were not usedas substrates 20 prior to grinding and flotation, the very fine metalsulfide particles would still end up in the flotation tails whenproducing concentrate 22. However, because these very fine sulfideparticles would not be partially biooxidized at this point, theiroccluded gold values cannot be recovered by lixiviation.

A fourth advantage of using refractory sulfide coarse ore as substrates20 is that the metal sulfide particles in the biooxidized supportmaterial will be easier to float following biooxidation. This is becausethe surface of the metal sulfide particles is altered during thebiooxidation process. Thus, after the coarse ore support material hasbeen reused several times and it is ground and floated to produce asulfide mineral concentrate, improved flotation results can be achieved.

If the coarse ore particles also contain a carbonate mineral component,a fifth advantage exists for using coarse refractory sulfide oreparticles as the coarse substrates 20. Carbonate minerals tend to bevery acid consuming. As a result, ores which contain these minerals havetraditionally required a lot of acid conditioning prior to biooxidation.Acid conditioning of these ores is required to remove or reduce thecarbonate mineral component prior to biooxidation so that thebiooxidation reaction can proceed. And, while coarse refractory sulfideore particles in general tend to biooxidize very slowly—often taking upto nine months or more—if lots of carbonate minerals are included in theore, without preconditioning, the coarse ore particles may neverbiooxidize. In the process according to the present invention, however,coarse refractory sulfide ore particles that contain carbonate mineralscan be advantageously used for substrates 20. During the biooxidationprocess, the acid produced from the biooxidation of the concentrate 22on the surface of the coarse ore substrates will slowly neutralize thecarbonate minerals in the substrates. A byproduct of the neutralizationprocess is carbon dioxide, which the autotrophic bacteria used in thepresent invention can use as a source of carbon to carry out metabolicsynthesis. The carbon dioxide production, therefore, will promotebacteria growth in heap 26, which in turn increases the rate ofbiooxidation of concentrate 22. Thus, by using coarse ore that containscarbonate minerals for support material 20, the coarse ore will beslowly neutralized for future biooxidation and bacteria growth in heap26 will be promoted. A concomitant benefit, as noted above, will be thebiooxidation of the very fine nonfloatable sulfide particles that are inthe coarse ore.

As those skilled in the art will recognize, the coarse refractorysulfide ore particles used for substrates 20 do not have to originatefrom the same ore body as that used to produce concentrate 22. In fact,in some situations, it may be beneficial to use a concentrate 22 fromone ore body and coarse ore substrates 20 from another. For example, oneore body may be easily concentrated or already have the characteristicsdesirable of a concentrate and another ore body may have a highconcentration of carbonate minerals. In such a situation, it would beadvantageous to use the first ore body to produce concentrate 22 and thesecond ore body to produce substrates 20. In this way, the ore from thesecond ore body can be neutralized in preparation for biooxidation whilesimultaneously improving the biooxidation results of the concentratefrom the first ore body. Similarly, if an ore body contains a highconcentration of metal sulfides that are difficult to float, improvedflotation results can be achieved by first using the ore as coarse oresubstrates 20 in the process according to the present invention.

Other preferred materials for substrates 20 include lava rock, gravel,and coarse rock containing carbonate minerals. These types of substrateswill typically be used when the refractory sulfide ore body being minedis a waste heap or tailings pile, and, as a result, the ore has alreadygone through crushing and grinding.

An advantage of using lava rock is that it has a very rough, nonuniformsurface morphology which increases the overall surface area of thesubstrates 20 for a particular particle size. Thus, for a given particlesize, lava rock can be loaded with more concentrate than othersubstrates having a smoother surface.

Gravel, while typically having a fairly smooth surface, is aninexpensive substrate material. Coarse rock containing carbonateminerals is advantageous, because it will slowly release carbon dioxideas the acid from the biooxidation process neutralizes the carbonateminerals as explained above. This type of substrate would preferably bereused in the process only as long as it continues to release carbondioxide during the biooxidation process.

A third embodiment of the present invention is now described inconnection with FIG. 3. The process according to the present embodimentis essentially a variation on the embodiment described in connectionwith FIG. 1. Accordingly, like items are referred to with the samereference numbers, and the description and considerations expressed withrespect to these items in connection with FIG. 1 will be understood toapply equally to the present embodiment.

As with the second embodiment, the process according to the presentembodiment can be used to liberate and recover precious metal valuesfrom a precious metal bearing refractory sulfide ore. For purposes ofthe present description, however, it is assumed that the sulfide mineralconcentrate 22 is produced from a gold bearing refractory sulfide ore.

According to the present embodiment, a plurality of substrates 20 arecoated with a sulfide mineral concentrate 22 in rotating drum 24 toproduce a plurality of coated substrates 39. The plurality of coatedsubstrates 39 are then stacked to form heap 26, which is used as a largenonstirred surface bioreactor.

The various considerations discussed above in connection with substrates20, sulfide mineral concentrates 22, the formation of coated substrates39, and the formation of heap 26 are all equally applicable here.

After heap 26 is formed the heap is inoculated with a biooxidizingbacteria to initiate the biooxidation process. As the biooxidationprocess proceeds, additional sulfide mineral concentrate 22 can be addedto the top of heap 26. An advantage of adding additional sulfide mineralconcentrate 22 to the top of heap 26 throughout the biooxidation processis that the amount of concentrate processed in the heap can be increasedbefore tearing down and rebuilding. Furthermore, if coarse refractorysulfide ore is used for substrates 20, concentrate 22 will tend tobiooxidize more quickly than the metal sulfide particles 40 found in thecoarse ore. Thus, by adding additional concentrate 22 to the top of heap26, the degree of biooxidation of the coarse ore substrates can beincreased before heap tear down. In addition, by adding the sulfidemineral concentrate 22 to the top of heap 26, acid and ferric ionsproduced during its biooxidation will migrate to the lower part of theheap where bacterial growth may be inhibited due to toxins, which havenot been washed from the ore early in the biooxidation process, or dueto the lack of oxygen. As a result, biooxidation of the sulfide mineralconcentrate and coarse ore substrates will proceed even if bacterialgrowth is not favored in this region.

There is another advantage to adding sulfide mineral concentrate 22 tothe top of heap 26 after it has been undergoing biooxidation for sometime, because such additions will increase the biooxidation rate in theheap. In the later stages of biooxidation of the coated substrates 39,most of the exposed and reactive sulfides will have already beenoxidized, resulting in a slow down in the rate of biooxidation. Thisslow down in the rate of biooxidation can lead to a drop in iron levelsand an increase in pH within heap 26. Addition of fresh reactive sulfidemineral concentrate 22 to the top of heap 26 can restart an activebiooxidation process due to the high ferric levels produced from thebiooxidation of the added concentrate, which in turn will increaseindirect chemical leaching of the sulfide mineral concentrate 22 coatedon substrates 20 and of metal sulfide particles imbedded in coarse oresubstrates 20.

Fresh concentrate 22 can be added to the top of heap 26 until the flowchannels within the heap begin to become plugged with the concentrateand biooxidized residue from the concentrate.

A second variation in the present embodiment from that in FIG. 1 is withrespect to how the precious metal values are recovered from the heapfollowing biooxidation. In the present embodiment, instead of tearingdown the heap and then separating the biooxidized concentrate from theheap for gold extraction, gold is extracted from the biooxidizedconcentrate—and if a coarse ore substrate is used, from thesubstrates—by directly lixiviating the heap with a precious metallixiviant. Preferably the lixiviant is one that functions at a low pH,such as thiourea, so the heap does not need to be neutralized prior tolixiviation. Furthermore, by using thiourea or other acid compatiblelixiviant, the liberated gold values can be extracted from the heap onan intermittent basis. For example, heap 26 can be biooxidized for aperiod, liberated gold values extracted with an appropriate lixiviant,and then the biooxidation process resumed. A fresh concentrate 22 ispreferably added to the top of heap 26 in slurry form with theresumption of the biooxidation process.

Gold is extracted from heap 26 by first allowing the bioleachatesolution to drain from the heap to acid circuit 30 following a desireddegree of biooxidation. After the heap is drained, an acid compatiblelixiviant such as thiourea is pumped from the lixiviant supply 38 to thesprinkler system 28 where it is dispersed onto heap 26. As the lixiviantpercolates through the heap, it dissolves liberated gold values from thesulfide mineral concentrate 22 and coarse ore substrates. The loadedlixiviant then collects at drain 29 where it diverted from the acidcircuit to a gold removal process 44, which preferably comprisesadsorbing the dissolved gold onto activated carbon or a synthetic resin.The barren lixiviant is then recycled to the lixiviant supply 38 andgold is recovered from the loaded activated carbon or synthetic resin.Processes for stripping adsorbed gold values from activated carbon andsynthetic resin are well known in the art and need not be describedherein.

A process according to a fourth embodiment of the present invention isillustrated in FIG. 4.

FIG. 4 illustrates a process for liberating and recovering metal valuesfrom a sulfide ore. As the process according to the present embodimenthas certain similarities to the embodiment described in connection withFIG. 1, like items have been referred to with the same referencenumbers. Furthermore, the description and considerations expressed withrespect to these items in connection with FIG. 1 will be understood toapply equally to the present embodiment.

According to the present embodiment, a sulfide mineral concentrate 22 isfirst produced from a sulfide ore. Concentrate 22 is comprised of aplurality of fine metal sulfide particles 40 and fine particles of sandor other gangue material 42.

Many different sulfide ores can be used to produce sulfide mineralconcentrate 22. Foremost amongst the sulfide ores that can be treated inthe present process are sulfide ores that contain sulfide minerals ofbase metals such as copper, zinc, nickel, iron, molybdenum, cobalt, oruranium. The metal values of interest in these ores are present in themetal moiety of the sulfide mineral particles in the ore. The metalvalues which are liberated and recovered, therefore, will depend on thespecific sulfide minerals present in concentrate 22 produced from theore. For example, if the sulfide ore used to produce concentrate 22contains chalcocite, bornite, and/or chalcopyrite, then the metal valuesrecovered will be that of copper. On the other hand, if concentrate 22is a concentrate of sphalorite, the metal values recovered will be thatof zinc.

After concentrate 22 is produced, sulfide mineral concentrate 22 is thencoated on a plurality of substrates 20 to form coated substrates 39.This is accomplished as described in connection with FIG. 1 by adding aplurality of dry substrates 20 and a slurry of concentrate 22 torotating drum 24, or, alternatively, by adding a plurality of drysubstrates 20 and concentrate 22 to rotating drum 24 and then sprayingthe mixture with an aqueous solution. The plurality of coated substrates39 produced in rotating drum 24 are stacked to form heap 26, which formsa large nonstirred surface bioreactor.

The various considerations discussed above in connection with substrates20, sulfide mineral concentrates 22, the formation of coated substrates39, and the formation of heap 26 are all equally applicable here.

After heap 26 is formed, the heap is inoculated with a biooxidizingbacteria to initiate the biooxidation process. As the metal sulfideparticles 40 in concentrate 22 biooxidize, the metal moiety of thesulfide particles dissolves in the bioleachate solution as it percolatesthrough the heap. After the bioleachate solution percolates through theheap, it is collected at drain 29. The bioleachate solution is thenprocessed to recover one or more desired base metal values by removingthem from the bioleachate solution using techniques well known in theart.

Following recovery of the desired metal values from the bioleachatesolution, the solution can be processed in acid circuit 30 to remove anyexcess toxins as described in connection with FIG. 1 and then reappliedto the top of heap 26.

Once the biooxidation reaction has reached an economically defined endpoint, that is after the metal sulfide particles 40 on the surface ofthe coarse substrates 20 are biooxidized to a desired degree, the heapis broken down and the biooxidized concentrate separated from the coarsesubstrates 20. The biooxidized concentrate is then disposed of in wasteor tailings pile 36. It is to be understood, however, that while thepresent embodiment has been described in terms of liberating andrecovering base metal values from the metal moiety of the metal sulfideparticles 40 in sulfide mineral concentrate 22, sulfide particles 40 canalso include occluded precious metal values. After biooxidation ofconcentrate 22, therefore, any precious metal values that are liberatedin concentrate 22 can be extracted and recovered as described inconnection with FIG. 1 prior to the disposal of the biooxidizedconcentrate.

The coarse substrates 20 which have been separated from the biooxidizedconcentrate can be recycled to the rotating drum for a new coating ofsulfide mineral concentrate 22. Alternatively, if coarse sulfide oreparticles are used for substrates 20, they are preferably processedafter one or more cycles through the process to form a sulfide mineralconcentrate of any metal sulfide particles 40 which remain unoxidized inthe coarse ore substrates. Sulfide mineral concentrate 22 is producedfrom the biooxidized coarse ore substrates as described in connectionwith the second embodiment.

A process according to a fifth embodiment of the present invention isillustrated in FIG. 5. The process illustrated in FIG. 5 is forliberating and recovering precious metal values from precious metalbearing refractory sulfide ores using a nonstirred bioreactor. Theprocess comprises producing a concentrate 22 of metal sulfide particles40 from the refractory sulfide ore being processed. Concentrate 22 isthen coated on a plurality of coarse substrates 20 to form coatedsubstrates 39 using rotating drum 24 as described in connection with thesecond embodiment. After formation, coated substrates 39 are placed in atank 45 for biooxidation. By biooxidizing substrates 39 in tank 45, alarge nonstirred surface bioreactor is created which has a very largesurface area. Thus, tank 45 takes the place of heap 26 in the processaccording to the second embodiment. Accordingly, the variousconsiderations discussed above in the second embodiment with respect tosubstrates 20, sulfide mineral concentrates 22, the formation of coatedsubstrates 39, and the formation of heap 26 are all equally applicableto the biooxidation of coated substrates 39 in tank 45 in the presentembodiment.

During the biooxidation of concentrate 22 on coated substrates 39,bioleachant maintenance solutions are added to the tank from the topusing any of a number of well known techniques. The bioleachate solutionthat percolates through the tank is drained from the tank and processedin acid circuit 30 as described in connection with FIG. 1 prior to reusein the process.

Air can be blown into the tank during the biooxidation process toimprove the oxygen levels in the bioreactor and to improve heatdissipation. Air is preferably blown into tank 45 through a series ofperforated pipes 46 which are connected to a blower (not shown).

If desired, additional concentrate 22 can be added to the top of thecoated substrates 39 in tank 45 throughout the biooxidation process. Asdescribed above in connection with the third embodiment, by addingadditional concentrate to the bioreactor during the biooxidationprocess, the rate of biooxidation within the bioreactor can bemaintained at a high level throughout the biooxidation process.

An advantage of using tank 45 over heap 26 for the bioreactor is that itmakes separation of the biooxidized concentrate 22 from the substrates20 easier. After the concentrate 22 is biooxidized to a desired endpoint, separation of the biooxidized concentrate from the substrates isaccomplished by filling the tank with water, and then rapidly drainingthe tank. The biooxidized concentrate will be carried with the drainingwater. This process can be repeated several times to improve separationresults. Tank 45 is also preferably equipped with a screen in the bottomof the tank which has a mesh size that is less than the size of thesubstrates, but larger than the concentrate particle size to aid theseparation process.

After separation, the biooxidized concentrate is leached with a preciousmetal lixiviant to extract the liberated gold or other precious metalvalues. The dissolved gold values are then recovered from the lixiviantby contacting the solution with activated carbon or a synthetic resin.Preferably the lixiviation is carried out in the presence of theactivated carbon or a synthetic resin so that the dissolved gold valuesare immediately removed from the solution as they are dissolved. Thegold adsorbed on the activated carbon or synthetic resins can berecovered using techniques well known in the art.

Once the precious metal values have been extracted from the biooxidizedconcentrate, the concentrate can be disposed of in waste or tailingspile 36.

As in the second embodiment, the coarse substrates 20 that have beenseparated from the biooxidized concentrates can be recycled to therotating drum for a new coating of sulfide mineral concentrate 22.Substrates 20 can be reused as long as they retain their mechanicalintegrity. If coarse refractory sulfide ore particles are used assubstrates 20, they are preferably processed at some point, preferablyafter one to three cycles, to recover liberated gold values. This isaccomplished in the same manner as described in connection with thesecond embodiment.

The preferred embodiments of the invention having been described,various aspects of the invention are further amplified in the examplesthat follow. Such amplifications are intended to illustrate theinvention disclosed herein, and not to limit the invention to theexamples set forth.

EXAMPLE 1

A sample of low grade (3.4 ppm) gold ore, which was known to berefractory to leaching with cyanide due to sulfides, was crushed. Theore was then separated into a −0.62 cm fraction (47.4 wt %) and a −0.31cm fraction (remainder). The −0.31 cm fraction was then further groundto 95% passing a 75 μm sieve to aid in producing a refractory pyriteconcentrate by flotation.

Water was added to the ground sample until it reached a 30% pulpdensity. The ore pulp was then adjusted to a pH of 10 and treated withNa₂SiO₃ at 6 Kg/tonne of ore for 12 hours to remove the clay material.The clay material was removed as the fraction that did not settle after12 hours.

Because clays can cause problems with flotation, a step that permits thenon clay material to settle out was added to remove the clay fractionbefore floating the sample.

The clay fraction was under 3% of the total ore weight, yet it containedalmost 5% of the gold in the ore. The removal and subsequent flotationof the clay fraction produced a very small weight fraction (0.1% of thetotal ore weight), but it contained over 17 ppm gold. Cyanide leachingof the clay flotation tail extracted over 76% of the gold containedtherein. The total amount of gold contained in the clay flotation tailwas 1.08 ppm.

Before floating, the main fraction of ground ore (+5 μm to −75 μm) wasconditioned with CaSO₄ at 2.0 Kg/tonne for ten minutes by mixing in aWemco flotation cell. This was followed by 10 minutes of mixing withXanthate at 100 g/tonne which was then followed by 5 minutes of mixingwith Dowfroth D-200 at 50 g/tonne. The sample was then floated for 20minutes at a pulp density of 30%. Four Kg of the main fraction wasprocessed in 8 separate batches of 500 g each. The sulfide concentratesobtained from these flotations were collected and combined and refloatedin a column.

Three fractions were collected, the tail from the Wemco float, the tailfrom the column float, and the sulfide concentrate, each of thesefractions were dried and weighed. The tail from the Wemco float was 35.4wt % of total ore weight and contained 1.88 ppm of gold. Cyanideleaching of this fraction yielded 67% of its gold. This was higher thanthe recovery for cyanide leaching of the whole ore, which was 63%. Thecolumn tail contained 3.56 ppm of gold. The gold recovery from thisfraction by cyanide leaching was 76.6%.

The sulfide concentrate weighed 753 g which represented 8.8% of thetotal ore (+0.31 cm and −0.31 cm fractions). Analysis of a smallfraction of the concentrate indicated it contained 6.5 ppm of gold. Thisfraction was coated on to the 47.4 weight percent of the +0.31 cm ore.The dry pyrite concentrate was spread over the surface of the coarse oreby rolling in a drum rotating at 30 rpm while spraying a mixture of2,000 ppm ferric ion and 1% Nalco #7534, which is an agglomeration aid.The pH of the solution was 1.8.

The mixture of concentrate on coarse ore support was placed into a 3inch column. Air and liquid were introduced from the top. The column wasinoculated with 10 ml of Thiobacillus ferrooxidans bacteria at an O.D.of 2.6 or about 1.1×10¹⁰ bacteria per ml.

The bacteria were grown in an acidic nutrient solution containing 5 g/lammonium sulfate and 0.83 g/l magnesium sulfate heptahydrate. The pH ofthe solution was maintained in the range of 1.7 to 1.9 by adjustmentwith sulfuric acid (H₂SO₄). The solution also contained iron at 20g/liter in the form of ferric and ferrous sulfate.

The bacteria were added to the top of the column after the pH wasadjusted to a pH of 1.8. The liquid, introduced to the top of the columnthroughout the experiment, was pH 1.8; with 0.2×9 K salts and 2,000 ppmferric. The extent of iron oxidation was determined by analysis of thesolution eluting off the column minus the iron introduced by the 2,000ppm ferric feed.

The composition of the standard 9 K salts medium for T. ferrooxidans islisted below. The concentrations are provided in grams/liter.

(NH₄)SO₄ 5 KCl 0.17 K₂HPO₄ 0.083 MgSO₄•7H₂O 0.833 Ca(NO₃)•4H₂O 0.024

The notation 0.2×9 K salts indicates that the 9 K salt solution strengthwas at twenty percent that of the standard 9 K salt medium.

After 60 days the amount of iron leached off of the column indicatedthat about 50% of the pyrite had been biooxidized. The experiment wasstopped and the mixture separated into a +600 μm fraction and a −600 μmfraction. Each fraction was ground to 95% minus 75 μm and then leachedwith a 500 ppm cyanide solution in a 96-hour bottle roll analysis.Activated carbon was added to the bottle roll test to absorb anydissolved gold.

The gold recovery of the −30 mesh fraction was 83.7%. The −30 meshmaterial had an increased head gold value of 8.87 ppm due to loss ofpyrite weight. The coarse +30 mesh fraction, on the other hand, had agold recovery of 57% and a head gold value of 2.24 ppm. This indicatedthat the concentrate pyrite that was coated on the outside of the coarserock had biooxidized faster than the coarse fraction of the rock.

EXAMPLE 2

Another comparative test was made. In this example, the biooxidationrates of ore size fractions were compared. The ore, which was providedby the Ramrod Gold Corporation, was crushed to 1.9 cm. The −0.31 cm orefraction was removed and used to form a concentrate. The ore sample hadless than 0.08 oz. of gold per ton of ore (2.7 ppm). The samplecontained both arsenopyrite and pyrite. The concentrate was made by ballmilling 5 Kg of the −0.31 cm inch ore until it passed −75 μm, the ballmilled ore was then floated with Xanthate to form a pyrite concentrate.Before flotation clay was removed by settling with Na₂SiO₃ at 6 Kg/tonneof ore for 8 hours or more. The flotation was done in small batches of500 g each in a laboratory Wemco flotation cell. Potassium Amyl Xanthatewas used as a collector at a concentration of 100 g/tonne along withsodium sulfide at 1.5 Kg/tonne and Dowfroth D-200 at 50 g/tonne. Thepyrite concentrate constituted 4.5% of the weight of the −0.31 cm orefraction. However, this ore fraction contained over 80% of the gold andpyrite for the milled ore. The concentrate contained approximately 17.4%iron, 15.7% sulfur and approximately 40 ppm gold. The +0.31 cm orecontained 0.9% iron and 0.18% sulfur.

A sample of 140 g of this concentrate was coated onto 560 g of +0.31 cmcoarse ore. The concentrate was added as a dry powder to the coarse ore.The mixture was then rotated in a small plastic drum at 30 rpm to spreadthe dry concentrate over the rock support. Liquid which contained 2,000ppm ferric ion and 1% Nalco #7534 was sprayed onto the mixture until allthe concentrate was coated onto the rock. The pH of the liquid wasmaintained at 1.8. The amount of liquid used was estimated to be between5 and 10 percent of the weight of the coarse ore and concentrate. The700 g mixture of concentrate on coarse ore substrates was placed into a3 inch column. The height of the ore after being placed in the columnwas approximately 5 inches. Air and liquid were introduced from the topof the column. The column of concentrate coated on coarse ore substrateswas inoculated with about 10 ml of bacteria at an O.D. of 2.0 or about8×10⁹ bacteria per ml.

The bacteria were a mixed culture of Thiobacillus ferrooxidans, whichwere originally started with ATCC strains #19859 and 33020. The bacteriawere grown in an acidic nutrient solution containing 5 g/l ammoniumsulfate and 0.83 g/l magnesium sulfate heptahydrate. The pH of thesolution was maintained in the range of 1.7 to 1.9 by adjustment withsulfuric acid (H₂SO₄). The solution also contained iron at 20 g/liter inthe form of ferric and ferrous sulfate.

The bacteria were added to the top of the column after the pH wasadjusted to pH 1.8. The liquid, introduced to the top of the columnthroughout the experiment had a pH of 1.8 with 0.2×9 K salts and 2,000ppm ferric ion. The extent of iron oxidation was determined by analysisof the solution eluting off the column minus the iron introduced by the2,000 ppm ferric feed.

This ore was low in sulfides having a concentration of less than 1% ofits weight. By making a concentrate on the coarse rock at 20% by weight,the concentration of both the pyrite and gold could be increased by overtenfold. This increased the rate of biooxidation as seen in FIGS. 6 and7 over that for the whole ore. Not only did this process expose more ofthe pyrite to air and water but it also increased the amount of ferricion and acid generated per unit volume of ore in the column model for aheap.

FIG. 6 shows the amount of oxidation as determined by percent ironleached for both the pyrite concentrate of this ore on +0.31 cm coarseore and the whole ore itself. As the graph shows the concentrate processwas biooxidized to about 40% in the first 30 days and over 65% in thefirst 60 days. Whereas the whole ore was only biooxidized to 24% in 84days. The average daily biooxidation rates are shown in FIG. 7. Thehighest average daily rate of the coated concentrate was 1.8% per daycompared to an average daily rate of only 0.5% for the whole ore. AsFIG. 7 illustrates, the coated concentrate sample did not take as longto begin biooxidizing the sample. This means that the coated concentrateprocess is more likely to achieve complete biooxidation in a reasonablyshort time.

Table 1 below shows the specific data points graphed in FIGS. 6 and 7for the concentrate on coarse ore process and for the whole ore processwhich was done for comparison.

After 68 days the concentrate coated on coarse ore column was takendown. The biooxidized material was separated into a plus 180 μm fractionand a minus 180 μm fraction. The weight of the fine material hadincreased from 140 g to 150 g. The total amount of iron removed from thesystem during the 68 days of biooxidation was 21.5 g which represents 46g of pyrite. The weight of the coarse rock decreased by 54 g. This wasbelieved to be due to breakdown of the rock to finer material due to thebiooxidation process. The total weight after biooxidation was 656 gwhich was 44 g less than the starting material. This fit well with theestimated 46 g of pyrite oxidized.

TABLE 1 Concentrate Process Whole Ore Process % Fe % Fe # of Daysleached % Fe/day # of days Leached % Fe/day 0 0.0 0.00 0 0.0 0.00 9 8.40.93 13 0.2 0.01 16 18.5 1.44 21 2.5 0.29 20 25.5 1.76 28 5.1 0.38 2331.0 1.82 35 8.6 0.50 28 37.5 1.30 42 11.7 0.44 33 41.7 0.84 49 13.80.29 37 46.1 1.10 56 15.9 0.31 43 51.8 0.95 62 18.4 0.42 51 60.7 1.11 7021.5 0.39 58 66.7 0.86 77 23.1 0.23 65 70.9 0.60 84 24.3 0.16

Two samples of the −180 μm material and one sample of the +180 μmmaterial were leached with cyanide. To leach the samples, bottle rollswere done for 96 hours, the leachant was maintained at 500 ppm cyanide.The +180 μm coarse ore support rock was ground to 95%-75 μm before doingthe bottle roll. All bottle rolls were done with activated carbon in theleach solution.

Sulfide analysis of the minus 180 μm fraction after 68 days ofbiooxidation showed the sample still contained 8.8% sulfides which was56% of the starting level. This was a lower percent oxidation thanindicated by the iron leached off during the column experiment. The goldrecovery increased to 84.3% for the high grade (38 ppm)−180 μm fractionand 79.5% for the +180 μm low grade (3 ppm) fraction. This is asubstantial increase from the 45.6% recovery of the unoxidized ore.

EXAMPLE 3

A sample of 70% minus 75 μm gold ore from a mine in the DominicanRepublic was used to make a sulfide float concentrate. The ore samplewas obtained from the tailing pile at the mine that had already beenleached with cyanide. The ore sample still contained gold values of over2 g per tonne which were occluded within the sulfides and not directlyleachable by cyanide.

To form the sulfide concentrate, several kilograms of this sample werefurther ground to 95% minus 75 μm. The ground sample was then floated toform the sulfide concentrate. The flotation was done in small batches of500 g each in a laboratory Wemco flotation cell. Before flotation, theground ore sample was adjusted to a pulp density of 30%. The ore slurrywas then mixed with 1.5 Kg/tonne sodium sulfide (Na₂S) for 5 minutes atpH 8.5. Then potassium amyl Xanthate was added as a collector at 100g/tonne and mixed for 5 minutes. Next 50 g/tonne of Dowfroth D-200 wasadded and mixed for 5 minutes. Finally, air was introduced to produce asulfide concentrate that contained 17.4% iron and 19.4% sulfide byweight and 14 g of gold per tonne of concentrate. A plurality of coatedsubstrates were then made by coating 140 g of the sulfide concentrateonto 560 g of +0.31 cm −0.62 cm granite rock. The concentrate was addedas a dry powder to the granite rock. The mixture was then rotated in asmall plastic drum at 30 rpm to spread the dry pyrite over the supportmaterial. A liquid which contained 2,000 ppm ferric ion and 1% Nalco#7534 agglomeration aid was sprayed on the mixture until all the sulfideconcentrate was coated onto the wetted granite rock. The solution wasmaintained at a pH of 1.8.

The coarse rock in this case had no iron or gold value. The rock,however, contained a small amount of mineral carbonate which tended tokeep the pH high at first but also provided CO₂ as a carbon source forthe bacteria.

The 700 g of concentrate coated rock was put into a column. A 0.2×9 Ksalts and 2,000 ppm ferric ion solution having a pH of 1.6 wasintroduced through the top of the column at a flow rate of about 300ml/day. Then the column was inoculated with 10 ml of bacteria as inExample 2. After the pH of the concentrate coated rock substrate wasadjusted to a pH of 1.8, the pH of the influent was set at 1.8. Air wasalso introduced through the top of the column.

FIG. 8 graphically illustrates the percent of biooxidation as determinedby the percent of iron leached from the concentrate. The average dailypercentage of biooxidation was calculated and is listed in Table 2 andis graphically illustrated in FIG. 9. The percentage biooxidation wasdetermined by dividing the total iron removed by the total ironcontained within the concentrate. The rate of biooxidation was slow tostart as the pH was adjusted and the bacteria built up and adapted.However, after about two weeks the rate increased rapidly and reached amaximum after 30 days. By this time almost 50% of the total iron hadbeen biooxidized. The process continued with a gradual slowdown as theremaining pyrite was consumed. At the end of 64 days nearly 97% of theiron had been biooxidized. Even with the concentrate almost completelybiooxidized and the rate slowing down near the end of the process, theaverage daily rate was still near 1%/day. After 70 days the biooxidationwas stopped. The biooxidized concentrate was separated into a plus 180μm fraction and a minus 180 μm fraction. The weight of the biooxidizedconcentrate had decreased from 140 g to 115 g. The total amount of ironremoved from the system during the 70 days of biooxidation was 25.9 gwhich represents 55.5 g of pyrite. The weight of the granite rockdecreased by 98.8 g. This was believed to be due to a breakdown of thecalcium carbonate in the rock by the acid as well as the breakdown ofthe rock to finer material. The total weight decreased by 123.3 g whichwas 67.8 g more than predicted by biooxidation of pyrite alone.

TABLE 2 Time in Days % Bioox. % Bioox./Day 5 2.590 0.288 15 10.270 1.10022 24.970 2.100 27 37.250 2.450 32 49.700 2.490 36 58.610 2.230 4268.580 1.660 50 82.580 1.750 57 90.870 1.180 64 96.820 0.850

The sample of −180 μm material was leached with 500 ppm cyanide in abottle roll for 96 hours. The +180 μm granite rock was also leached with500 ppm cyanide to determine how much gold could be stuck to the supportrock in a process that used barren rock as a supporting substrate.Analysis of the −180 μm material showed it still contained 9.7% sulfidewhich indicated only about 50% oxidation.

Gold extraction was 77% from the −180 μm fraction. This gold wasrecovered from gold ore that had already been leached with cyanide, thusdemonstrating that the process according to the present invention iseven applicable to ores which heretofore have been considered waste. Andwhile any recovery would be an improvement over the process currentlypracticed at the mine, the process according to the present inventionwas able to recover 77% of the gold in what was previously consideredtailings.

Cyanide leaching of the granite support rock showed that it had pickedup 0.15 ppm of gold which was 3.4% of the total gold.

EXAMPLE 4

A sample of gold bearing refractory sulfide ore that had been crushed to80% passing 0.62 cm was prepared for testing as support rock. The orewas from the Western States mine located in Nevada, and contained a highconcentration of carbonate minerals in the form of limestone. The finematerial (less than 0.31 cm) was removed in order to allow for good airflow. A four kilogram sample of the +0.31 cm to −0.62 cm rock was coatedwith one kilogram of a gold bearing pyrite concentrate provided byanother mining company. The coating was formed by placing a the coarseore substrates and dry concentrate into a small rotating drum andspraying the mixture with a liquid which contained 2,000 ppm ferric ionand 1% Nalco #7534 agglomeration aid until all the sulfide concentratewas coated onto the wetted granite rock.

Iron analysis of both samples showed that the concentrate contained 210grams of iron and the four kilograms of support rock contained 42.8grams of iron.

The five kilograms of coated ore substrates was placed in a 3 inchcolumn. To start the biooxidation process, a solution having a pH of 1.3and containing 2,000 ppm of ferric ions was passed through the column atabout one liter per day. After seven days, the pH of solution leavingthe column was below pH 2.5. At this point the column was inoculatedwith 10 ml of a culture of Thiobacillus ferrooxidans bacteria (as inExample 2) and the pH of the feed solution was raised to a pH of 1.8.After a total of fifteen days the column was generating acid at a pH of1.7 and an Eh of 700 mV. The progress of the biooxidation process wasfollowed by measuring the iron leaching off the column of concentratecoated nominal 0.62 cm ore. This data was compared with the data from anexperiment using the same concentrate coated on a sample of barren rock.The rates of the leaching in both cases are compared in graph form inFIG. 10. The fact that the Western States experiment was slightly fastersuggests that the coarse ore support rock was also oxidizing to someextent.

The Western States column experiment ran for a total of 74 days andleached a total of 166 grams of iron out of the system or 66% of thetotal iron in both the concentrate and support rock. Most of the ironwas leached from the concentrate, but some came from the support rock.The weight of the concentrate changed from 1,000 grams to 705.8 gramsafter biooxidation. The four kilograms of Western States coarse oresupport rock decreased to 3695.5 grams, which corresponds to a loss of304.5 grams or 7.6% of its weight after biooxidation. The decrease inweight of the coarse ore support rock was due to a combination ofbiooxidation of its pyrite, acid leaching of the carbonate in the ore,and physical abrasion of the ore.

The 705.8 grams of biooxidized concentrate, which was originally fromanother mine in Nevada, was tested for gold extraction using a cyanidebottle roll test. The gold recovery before biooxidation was 46%. Afterbiooxidation it increased to 86%. This same gold recovery was achievedby biooxidizing the concentrate to the same extent on the gravel supportmaterial.

The acid consumption of the Western States ore was measured before andafter its use as a support rock for biooxidation. The amount of sulfuricacid required to adjust the pH down to 2 before biooxidation was 31.4 gper 100 g ore. The amount of acid required to adjust the pH down to 2after biooxidation was 11 g per 100 g ore. This would mean that about20% of the weight of the support rock was acid neutralized during the 74days of biooxidation. This was larger than the 7.6% loss in weight ofthe support rock. This may be due to a precipitate forming on the rockafter biooxidation or sample to sample variation in the percentlimestone.

Several conclusions can be drawn from this test. First, a low pHbiooxidation process can occur on the surface of a high carbonate ore.Second, with the +0.31 cm to −0.62 cm support material, the process ofneutralization by the pH 1.8 acid was slow enough that the carbonate inthe ore was still not completely removed after 74 days. The process ofslow acid neutralization is beneficial to the bacteria, because theneutralization of the limestone in the ore will provide needed CO₂ forthe biooxidizing bacteria's carbon source. Third, the coarse ore supportwas benefited from the process because smaller nonfloatable sulfides inthe Western State ore were biooxidized.

Based on the amount of neutralization that occurred in about 2 months ofthe +0.31 cm to −0.62 cm coarse ore support, a +0.62 cm to −1.9 cmcoarse ore support rock would be best for a full scale process. With thelarger coarse ore support, it will take 90 to 120 days in a heapbiooxidation process to make the best use of the limestoneneutralization and to biooxidize the smaller floatable sulfides in thecoarse ore support rock. The time it takes to biooxidize the coating ofsulfide spread on the outside of the coarse ore support is generallyless than 90 days. Therefore, the coarse ore support may be used severaltimes before it is ground up and floated to make a pyrite concentratefor biooxidation on the surface of a coarse ore support rock.

Prior to biooxidation, two attempts were made to produce a concentrateby flotation of the Western States ore. One method used only xanthateand produced only a small recovery of gold (less than 12%) into thepyrite concentrate. The tail from this flotation still contained 4.0 gAu/tonne. Extraction of the flotation tail with cyanide only recovered17% of the gold remaining in the tail.

A second attempt at flotation used both kerosene to float off a carbonconcentrate followed by xanthate to produce a pyrite concentrate. Thecombined weight of these concentrates accounted for 18 weight % of theore, which was double the 7.4 weight % concentrate produced using onlyxanthate. The combined gold recovery for both concentrates increased to53.8% of the gold. The tail from this flotation decreased to 2.12g/tonne in gold. Extraction of the tail with cyanide recovered only34.5% of the gold remaining in the tail after flotation of bothconcentrates.

The third attempt at flotation was done with the Western States oreafter it had been used as a support rock for biooxidation in the presentexample. The +0.31 cm to −0.62 cm ore substrates were ground to −75 μmand then floated using xanthate as a collector. This formed a pyriteconcentrate of 33.4 g Au/tonne and 7.9% of the original ore weight. Thetail from this flotation contained 1.09 g Au/tonne. The recovery of thegold into the pyrite concentrate was 72.4%. Cyanide extraction of the1.09 g/tonne tail recovered 48.7% of the gold to produce a final tail of0.56 g/tonne.

The 33.4 g Au/tonne pyrite concentrate was biooxidized in a shake flaskexperiment. After biooxidation the cyanide extraction had increased to99% gold recovery. This result showed that this concentrate was goldcontaining pyrite that could be biooxidized along with other concentratein the coated substrate process.

As can be seen from the flotation results contained in Table 3 below, byfloating the Western States ore after it was used as a support materialfor biooxidation, a high grade pyrite concentrate was more easilyproduced, and the flotation tail was less refractory to cyanideextraction. This may have been due to a chemical change to the pyriteduring the 74 days in the high ferric and low pH conditions ofbiooxidation. Alternatively, the nonfloating sulfides may have been madeless refractory by a combination of ferric and bacterial oxidation

TABLE 3 FLOTATION RESULTS 3rd after bioox. 1st Pyrite Float 2nd floatfloat grinding −75 μm −75 μm −75 μm reagents for flotation XanthateKerosene NaS, CuSO4 Dowfroth NaSiO3 Xanthate Xanthate Dowfroth DowfrothWt. % of pyrite conc.  7.4%  3.2%  7.9% Wt. % of carbon conc. — 14.8% —Total wt. % of conc.  7.4% 18.0%  7.9% Grade of conc.  6.4 g/t 26.4 g/t33.4 g/t % gold in conc. 11.3% 53.8% 72.4% Gold in tail  4.0 g/t 2.12g/t 1.09 g/t before CN Gold in tail 3.32 g/t 1.39 g/t 0.56 g/t after CNGold recovery from 17.2% 34.5% 48.7% leaching tail by CN Combined total26.4% 69.2% 85.4% recovery Head grade of sample 4.18 g/t 3.77 g/t 3.64g/t tested

Although the invention has been described with reference to preferredembodiments and specific examples, it will readily be appreciated bythose of ordinary skill in the art that many modifications andadaptations of the invention are possible without departure from thespirit and scope of the invention as claimed hereinafter. For example,while the processes according to the present invention have beendescribed in terms of recovering gold from refractory sulfide orrefractory carbonaceous sulfide ores, the processes are equallyapplicable to other precious metals found in these ores such as silverand platinum. Similarly, the process according to the present inventionmay, as one skilled in the art would readily recognize, be used tobiooxidize sulfide concentrates from metal sulfide ores such aschalcopyrite and sphalorite.

1. A method of biotreating and recovering metal values frommetal-bearing refractory sulfide ore using a nonstirred surfacebioreactor, said method comprising the steps of: a. producing aconcentrate of metal sulfide particles from the refractory sulfide ore,b. coating the surface of a plurality of coarse substrates having aparticle size of greater than about 0.3 cm with the metal sulfideparticles to be biotreated and thereby forming a plurality of coatedcoarse substrates, said metal sulfide particles to be biotreated havinga particle size of less than about 250 μm; c. forming a nonstirredsurface reactor by stacking said plurality of coated coarse substratesinto a heap or placing said plurality of coated coarse substrates into atank, said reactor having a void volume greater than or equal to about25% and a surface area of greater than or equal to 100 square meters percubic meter of reactor space; d. biooxidizing the metal sulfideparticles on the plurality of coarse substrates; e. contacting thebiooxidized metal sulfide particles with a metal lixiviant to therebydissolve metal values from the biooxidized metal sulfide particles; andf. recovering precious metal values from the lixiviant.
 2. A methodaccording to claim 1, wherein said plurality of coarse substrates arerock, and said rock is selected from the group consisting of lava rock,gravel, and barren rock containing carbonate minerals.
 3. A methodaccording to claim 1, wherein no more than 5% by weight of said coarsesubstrates are less than 0.3 cm.
 4. A method according to claim 1,wherein the surface area of the reactor per cubic meter of reactor spaceis greater than or equal to 500 square meters per cubic meter ofreactor.